Her mother bore her from a marriage to a Christian man, and she is asking about the ruling on her relationship with her father and her non-Muslim sisters
Question

 
I am a muslim woman living in a non Islamic country.
My mother is a Muslim but unfortunately she married a Christian in a secular cour, t lived together with him for about twenty years and has seven children with him including me. AlhamduliLlah now they are separated.
In terms of religion, we were told that when we grow up every child would choose their religion. Nevertheless we were raised going to church mostly and celebrate both Islamic and Christianity celebrations. Our mother was telling us Islam is the truth but she was not actively teaching nor was she practicing it.
AlhamduliLlah some of us became Muslims while two of our sisters after reaching puberty they got baptized and got married to Christians.
Personally I did not attend any of their weddings and I do not interact with them at all. This is due to my understanding that if a child is born from a Muslim mother then she automatically becomes a Muslim and hence if she decides to be baptized after puberty she has then retarded
On the other side my mother and my other siblings are treating them nicely, seeking assistance from them and cooperating with them well as one would do to her Muslim children and sisters.
Please clarify to me:
1. How should we interact with our non Muslim sisters and their family according to the Qur’an and Sunnah?
2. What are our responsibilities towards the man who raised us as father and if we are allowed to take assistance from him or from my Christian sisters?
Praise be to Allah.Firstly: 
It is haraam for a Muslim woman to marry a disbeliever, and there is no difference of opinion among the Muslims concerning that. It is one of the major sins that incur the wrath of the Knower of the unseen. Allah, may He be exalted, says (interpretation of the meaning):
“And give not (your daughters) in marriage to Al-Mushrikoon till they believe (in Allah Alone) and verily, a believing slave is better than a (free) Mushrik (idolater, etc.), even though he pleases you”
[al-Baqarah 2:221]
“O you who believe! When believing women come to you as emigrants, examine them, Allah knows best as to their Faith, then if you ascertain that they are true believers, send them not back to the disbelievers, they are not lawful (wives) for the disbelievers nor are the disbelievers lawful (husbands) for them”
[al-Mumtahinah 60:10]. 
If that marriage has taken place, then it is invalid. The one who is aware that it is haraam but regards it as permissible is undoubtedly a disbeliever. 
For more information on this matter, please see fatwa no. 170862 
Secondly: 
The fuqaha’ have stated that the child should follow the better of the parents’ religions. One such scenario is if a Muslim man marries a Jewish or Christian woman, then in that case the child will be Muslim. Another example is that which was mentioned by al-‘Allaamah Ibn ‘Aabideen al-Hanafi, who said: The child should follow the better of his parents’ religions. This may be imagined in a case of conversion to Islam, where both spouses were disbelievers, then he became Muslim or she became Muslim, then a child was born before the other partner became Muslim.
End quote from ad-Durr al-Mukhtaar wa Haashiyat Ibn ‘Aabideen (3/196) 
It says in al-Bahr ar-Raa’iq Sharh Kanz ad-Daqaa’iq (2/205): One of his parents became Muslim, so he is to be regarded as Muslim, following his parent, whether the child has reached the age of discernment or not, because the child follows the better of his parents’ religions. End quote. 
If it so happens that a Muslim woman marries a disbeliever, then even though this marriage is invalid, the children follow their mother in being Muslim. It says in Badaa’i‘ as-Sanaa’i‘ fi Tarteeb ash-Sharaa’i‘ (7/139): If an apostate marries a Muslim woman and she bears him a boy, or he has intercourse with a Muslim slave woman and she bears him a child, then the child is Muslim, following his mother, and may inherit from his father because the blood ties between them are proven. If the mother is a disbeliever, then he is not deemed to be Muslim, because neither of the parents was Muslim. End quote. 
Thirdly: 
If it so happens that a Muslim woman married a disbeliever, if they both believed that this marriage was valid, because they were ignorant of its prohibition in the laws of Allah, then children of this marriage are to be regarded as legitimate, because they did not know the ruling on the marriage contract and they believed it to be valid. But if they were aware that this marriage was prohibited, then the children are not to be regarded as legitimate. 
Ibn Qudaamah (may Allah have mercy on him) said: 
If a woman who is still in ‘iddah (waiting period following death of the husband or divorce) gets married and they are both aware of the ‘iddah and the prohibition on getting married and having intercourse during the ‘iddah, then they are both zaanis (fornicators, adulterers); they are both to be subjected to the hadd punishment, she is not entitled to any mahr, and any offspring is not to be attributed to him. If they were both unaware of the ‘iddah or the prohibition, then any offspring is to be regarded as legitimate, the hadd punishment is not to be carried out, and she must be given a mahr. If he was aware of that but she was not, then he is to be subjected to the hadd punishment and he is obliged to give the mahr, but any offspring is not to be attributed to him. If she was aware of it but he was not, then she is to be subjected to the hadd punishment and she is not entitled to any mahr, and any offspring is to be attributed to him. That is the case because this is a marriage concerning which there is scholarly consensus that it is invalid, so it is akin to marriage to mahrams.
End quote from al-Mughni (8/127) 
See also: Kashf al-Qinaa‘ (13/44), al-Wizaarah edn; Mataalib Ooli an-Nuha (5/579). 
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) was asked about a man who divorced his wife three times; a mufti gave him a fatwa stating that the divorce did not count as such, and the husband followed that fatwa and had intercourse with his wife after that, and she bore him a child, and it was said that he was an illegitimate child. 
He replied: 
The one who said that is extremely ignorant and misguided, and in opposition to Allah and His Messenger, because the Muslims are unanimously agreed that in the event of any marriage which the husband believes to be a valid marriage, if he has intercourse with the wife, then the child is to be attributed to him, and father and son may inherit from one another, according to the consensus of the Muslims, even if that marriage is in fact invalid according to the consensus of the Muslims, and regardless of whether the man is a disbeliever or a Muslim. 
If a Jew marries the daughter of his brother, any child born as a result of that is to be attributed to him and may inherit from him, according to the consensus of the Muslims, even though that marriage is invalid according to the consensus of the Muslims.
To read this fatwa in full, see: Majmoo‘ al-Fataawa (34/14ff) 
Abu Haneefah favoured the view that the child is to be attributed to the father in all cases, even if both partners were aware of the prohibition, because there appeared to be a marriage contract; his view differed from that of his two companions. 
See: al-Mabsoot by as-Sarkhasi (17/133); Ahkaam adh-Dhimmiyyeena wa’l-Musta’maneena fi Dar al-Islam, by Dr. ‘Abd al-Kareem Zaydaan (291-29 2). 
Fourthly: 
From the above it is known that your two sisters who chose Christianity and were baptised are apostates from Islam, and it is not permissible to love or take as a friend one who has apostatised from Islam; rather it is obligatory to despise and disavow him, because Allah, may He be exalted, says (interpretation of the meaning):
“You (O Muhammad blessings and peace of Allah be upon him) will not find any people who believe in Allah and the Last Day, making friendship with those who oppose Allah and His Messenger (Muhammad blessings and peace of Allah be upon him), even though they were their fathers, or their sons, or their brothers, or their kindred (people). For such He has written Faith in their hearts, and strengthened them with Rooh (proofs, light and true guidance) from Himself.”
[al-Mujaadilah 58:22]
And Allah, may He be exalted, tells us of what Nooh (peace be upon him) said concerning his son who was a disbeliever (interpretation of the meaning):
“And Nooh (Noah) called upon his Lord and said, ‘O my Lord! Verily, my son is of my family! And certainly, Your Promise is true, and You are the Most Just of the judges.’
He said: ‘O Nooh (Noah)! Surely, he is not of your family; verily, his work is unrighteous, so ask not of Me that of which you have no knowledge! I admonish you, lest you be one of the ignorant’”
[Hood 11:45, 46].
And Allah, may He be exalted, tells us about Ibraaheem (peace be upon him):
“And (Ibraheem’s (Abraham)) invoking (of Allah) for his father’s forgiveness was only because of a promise he (Ibraheem (Abraham)) had made to him (his father). But when it became clear to him (Ibraheem (Abraham)) that he (his father) is an enemy to Allah, he dissociated himself from him. Verily Ibraheem (Abraham) was Al-Awwah (has fifteen different meanings but the correct one seems to be that he used to invoke Allah with humility, glorify Him and remember Him much), and was forbearing”
[at-Tawbah 9:114]. 
But it is permissible for you to treat them kindly with the aim of softening their hearts so that they might return to Islam, on condition that you are confident that you will not be tempted or confused by inclining towards them or towards the falsehood that they are following. 
It says in Fataawa al-Lajnah ad-Daa’imah 1 (2/67):
It is not permissible to make friends with the disbelievers or to mix with them in such a way that may lead to confusion. As for eating with them, and mixing with them and treating them kindly in a way that may encourage them to become Muslim, there is nothing wrong with that, so long as one is free of the risk of confusion and does not love them. End quote. 
See also the answer to question no.  169985  
Fifthly: 
With regard to your father, what you have to do is treat him kindly. Allah, may He be blessed and exalted, says (interpretation of the meaning):
“And We have enjoined on man (to be dutiful and good) to his parents. His mother bore him in weakness and hardship upon weakness and hardship, and his weaning is in two years give thanks to Me and to your parents, unto Me is the final destination.
But if they (both) strive with you to make you join in worship with Me others that of which you have no knowledge, then obey them not, but behave with them in the world kindly, and follow the path of him who turns to Me in repentance and in obedience”
[Luqmaan 31:14, 15]. 
Ibn Katheer (may Allah have mercy on him) said:
That is, if they strive hard to make you follow them in their religion, then do not accept that from them, but do not let that prevent you from behaving with them in the world kindly, that is, showing kindness to them, “and follow the path of him who turns to Me in repentance and in obedience,” i.e., the path of the believers.
End quote from Tafseer Ibn Katheer (6/337) 
Another indication that it is permissible to uphold ties with a father who is a disbeliever is a report that was narrated from Asma’ bint Abi Bakr (may Allah be pleased with her), who said: I said (to the Prophet (blessings and peace of Allah be upon him): My mother has come to me and she is expecting (something), should I uphold the ties of kinship with my mother? He said: “Yes, uphold the ties of kinship with your mother.”
Narrated by al-Bukhaari (2477) and Muslim (1003). 
And you should try your hardest to call him to Islam and soften his heart towards it. 
See also the answers to questions no.  95588 and 27196. 
Sixthly: 
Seeking help from a disbeliever, whether he is a relative or otherwise, is subject to further discussion. If there is no fear that you will be inclined towards him or become unduly fond of him because of his kindness, then there is nothing wrong with seeking help from him and accepting it. But if there is the fear that this will lead to you being inclined towards him or being confused by the falsehood that he is following, then in this case it is not permissible to accept their help or to ask for it in the first place. Rather you should seek help from your Muslim brothers. Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him) was asked: What is the ruling on seeking help from the disbelievers and accepting it? 
Answer: That is subject to further discussion. If there is no fear that seeking their help and accepting it will be detrimental to the religious commitment of the one who asks for that or accepts it, then there is nothing wrong with it. If there is some such risk involved, then it is not permissible to ask for it or accept it, acting in accordance with the shar‘i evidence which indicates that it is essential to take precautions to avoid that which Allah has prohibited and to keep away from that which incurs the wrath of Allah. 
It is proven that the Prophet (blessings and peace of Allah be upon him) accepted gifts from some of the mushrikeen, but not from others, and the reason for that was what we have mentioned here, as has been stated by the scholars. 
And Allah is the source of strength; may Allah send blessings and peace upon our Prophet Muhammad and upon his family and companions. 
End quote from the Shaykh’s website on the following link (in Arabic)
And Allah knows best.