Does a mother have to be fair in giving gifts to her children?
Question

I used to give my mother a thousand riyals every month, despite the fact that she did not need the money. That was an attempt on my part to do some small act of kindness to my parents. She used to save this money, and after a while I started to build a house just for me and I needed that money, so I stopped giving her that monthly amount. Then my mother suggested that I and my (married) sister who is not well off should share the money that belongs to her. Please note that I have a brother and another sister (who is also married) who are living a comfortable life. 
My question is: 
Is it permissible for me and my needy sister to share this money without giving any to our brother and other sister? If it is not permissible, then how should my mother share out this money?.
Praise be to Allah.Firstly: 
Allaah has enjoined fairness when giving to children, both males and females. 
It was narrated from al-Nu’maan ibn Basheer that his father brought him to the Messenger of Allaah (peace and blessings of Allaah be upon him) and said: “I have given this son of mine a slave.” He said, “Have you given a similar gift to all of your children?” He said, “No.” He said, “Then take it back.” Narrated by al-Bukhaari (2446) and Muslim (1623). 
With regard to a parent favouring some children over others in giving, he or she should be fair in giving to them, either by taking back the gift, or giving gifts to the others so that they are all treated fairly. 
See al-Mawsoo’ah al-Fiqhiyyah, 11/359. 
Secondly: 
The prohibition on favouring one child over another in gift-giving applies to the mother just as it applies to the father, because the Prophet (peace and blessings of Allaah be upon him) said: “Fear Allaah and treat your children fairly.” And because she is one of the two parents, and she is forbidden to favour one child over another just as the father is. If the father shows favour to some of the children, that generates hatred and resentment. The same applies if the mother shows favour to some of her children over others. End quote. Al-Mughni, 8/261. 
Thirdly: 
The way in which gifts should be shared out among children, according to sharee’ah, is the same as the way in which inheritance is shared out, with the male getting the share of two females, because there is nothing more just than the way of dividing that is prescribed by Allaah. 
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said in al-Ikhtiyaaraat (p. 184): 
It is obligatory to treat children justly in gift-giving, along the same lines as the shares of inheritance. This is the view of Ahmad. End quote. 
Shurayh al-Qaadi said to a man who shared out his wealth among his children: The way in which Allaah has shared it is more just than the way in which you have shared it. Take it back and share it out in the manner decreed by Allaah. Narrated by ‘Abd al-Razzaaq in his Musannaf. 
‘Ata’ said: “They only used to share out (wealth) according to the Book of Allaah.  
See al-Mughni, 8/261 
It says in Fataawa al-Lajnah al-Daa’imah (16/197): 
If your father wants to share out his wealth or some of his wealth among his children, he should share it out among males and females in accordance with the shares of inheritance prescribed by sharee’ah, whereby the male gets the share of two females. End quote. 
Fourthly: 
If the children are adults of sound mind, it is permissible for the father or mother to give one of their children more/less than others, so long as that is with their agreement and without any pressure. 
Shaykh Ibn Baaz (may Allaah have mercy on him) said:  
The father must be just and fair to his children, males and females, along the lines of the shares of inheritance. It is not permissible for him to give gifts to some of them and not to others, except with the consent of those who are not given, if they are mature. Their consent should not be given out of fear of their father, rather it should be given willingly, with no threats or fear of their father. Not differentiating between them is better in all cases, and is better for their hearts, because the Prophet (peace and blessings of Allaah be upon him) said: “Fear Allaah and treat your children justly.” Saheeh – agreed upon. End quote. 
Majmoo’ Fataawa al-Shaykh Ibn Baaz, 9/452 
The Shaykh (may Allaah have mercy on him) also said: 
There is no doubt that some children may be better than others, this is something that is well known, but the father has no right to differentiate between them because of that. Rather he must be fair, because the Prophet (peace and blessings of Allaah be upon him) said:  “Fear Allaah and treat your children justly.” So it is not permissible for him to differentiate between them because one is better or honours his parents more than another. Rather he must treat them fairly and be sincere towards all of them, so that they will be kind to and honour their parents and obey Allaah and His Messenger. But he should not differentiate between them in gift giving, or leave money to some of them and not to others; rather they should all be treated fairly in terms of inheritance and gift-giving, and he should treat them fairly as prescribed in sharee’ah, with a male being given the share of two females. If a male child is given a thousand, a female should be given five hundred. If they are mature and easy-going, and they say, “Give our brother such and such,” and they clearly state that they do not mind if he is given a car or whatever, and it is clear that their tolerance is genuine and is not done under pressure or because they are afraid of him, then there is nothing wrong with that. 
What matters is that they should be treated fairly, unless the children are mature, whether males or females, and they do not mind if one of them is given something extra because of special circumstances, in which case there is nothing wrong with that and they have the right to do that. End quote. 
Majmoo’ Fataawa al-Shaykh Ibn Baaz, 9/235. 
Based on this, what your mother must do is to share this wealth fairly among you, giving each male the share of two females. If some of you are given more with the approval of the others, there is nothing wrong with that in sha Allaah. 
And Allaah knows best.