Her husband’s sister is causing problems for them
Question

 
I am a married woman and mother of three children, and my husband fears Allah and honours his parents. But my husband’s sister always causes problems between me and my husband, because she tells him a lot of lies which always causes us to argue and sometimes affects our marital life. But recently my husband has found out that she is in fact lying. This time things got very complicated, as she had the audacity to swear and revile and impugn my honour and that of my husband, and she incited his parents against him and me by fabricating lies so as to demonstrate her innocence. It should be noted that she is their only daughter, which gives her parents more reason to believe what she says, so they believed her and did not believe my husband. This has led to tension between my husband and his parents, and with me too. But despite all this he still upholds ties with his parents and his other siblings, except for her. 
Is there any sin on my husband for severing ties of kinship with her? Is she regarded as kin for me? Please note that I have tried in many ways, direct and indirect, to get close to her, such as giving her gifts, honouring her as a guest and so on, but now I want to keep away from her so as to avoid trouble between me and my husband’s family, so that I will not be a barrier between my husband and his parents. With regard to his parents, I want to remain in touch, but from a distance, such as getting in touch on special occasions. May Allah reward you with good.
Praise be to Allah.I ask Allah to set your affairs straight and not to allow the shaytaan any way to affect you, and to guide you to that which is good and right. 
You have done very well by hastening to reconcile and work things out, and in this way you have done what is required of you and avoided sin. Your efforts will be accepted in sha Allah. After doing all you could to try to bring about a reconciliation and solve the problem, keeping your distance may be the best solution, I mean your keeping away from your husband’s sister and avoiding close contact with her, especially if the matter has reached the stage of impugning people’s honour. The same applies to keeping away from your husband’s parents, if keeping in touch will cause problems, because the husband’s family are not close relatives (arhaam) of yours with whom you are obliged to uphold ties, as has been explained in the answer to question no. 75057. 
Rather they are the husband’s relatives, so it is not permissible for him to cut off ties with them, even if they annoy him and insult him.
 It was narrated from Abu Hurayrah (may Allah be pleased with him) that a man said:  O Messenger of Allah, I have relatives with whom I try to keep in touch, but they cut me off. I treat them well, but they abuse me. I am patient and kind towards them, but they insult me. He said: “If you are as you say, then it is as if you are putting hot ashes in their mouths. Allah will continue to support you as long as you continue to do that.” 
Narrated by Muslim (2558). 
Al-Nawawi (may Allah have mercy on him) said: 
What this means is that it is as if you are feeding them hot ashes, which is a simile, likening the pain they feel to the pain felt by one who eats hot ashes, and there will no blame on the one who is doing the right thing, rather they are the ones who are committing the great sin of severing ties with him and annoying him. And it was said that what is meant is that by treating them kindly, you will make them feel ashamed of themselves because of the greatness of your kindness towards them and the abhorrent nature of their actions. And it was said that what they consume of your kindness is like hot ashes that burn their stomachs. Sharh Muslim (16/115). 
So he has to put up with all the annoyance he faces from them, strive to uphold ties with them within the limits that will not cause further problems, and try not to make matters worse. He should use kindness and good treatment in dealing with his sister, in the hope that Allah may guide her to mend her ways. He should try to advise her and explain how things really are, and he may seek the help of others who are closer to her than him. He should treat his parents with nothing but kindness, and you should help him in that and encourage him to uphold ties with them and treat them kindly. Remind him of the verse in which Allah says (interpretation of the meaning): 
“The good deed and the evil deed cannot be equal. Repel (the evil) with one which is better (i.e. Allah orders the faithful believers to be patient at the time of anger, and to excuse those who treat them badly) then verily he, between whom and you there was enmity, (will become) as though he was a close friend.
35. But none is granted it (the above quality) except those who are patient — and none is granted it except the owner of the great portion (of happiness in the Hereafter, i.e., Paradise and of a high moral character) in this world” [Fussilat 41:34-35]
The scholars of the Standing Committee were asked about a similar problem to that mentioned in the question, and they replied: 
We advise you to honour your mother and to speak kindly to her, and not to show that you are upset with her. You must keep on advising your sister in good ways to refrain from stirring up problems if she is not in the right. 
Fatawa al-Lajnah al-Daa’imah (25/254). 
Let your intention in keeping away from them be to wait until everyone has calmed down and can think again, until Allah creates love and compassion among you and good relations are restored in your family, for that is one of the greatest aims which sharee’ah came to achieve among people. 
It says in Fatawa al-Lajnah al-Daa’imah (26/127): 
Islam enjoins instilling love among the Muslims, and encourages them to love one another, be compassionate towards one another, and uphold ties with one another, so that their affairs will be set straight, their hearts will be pure and they will be united against others. Islam warns them against enmity and grudges, and forbids them to forsake one another and sever ties. It is haraam to sever ties with another Muslim for more than three days. In al-Saheehayn and elsewhere it is narrated from Abu Ayyoob al-Ansaari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “It is not permissible for a Muslim to forsake his brother for more than three days, each of them turning his face away when they meet. The better of them is the one who greets the other first.” In Sunan al-Tirmidhi it is narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Beware of discord, for it is the shaver,” i.e., it shaves religious commitment. 
What the Muslim must do if there is any problem between him and his brother is to go to him, and greet him with salaam, and treat him kindly in order to reconcile between them. There is great reward and salvation from sin in that. 
End quote. 
And Allah knows best.