The parents have to treat their children fairly when giving gifts, because of the report narrated by al-Bukhaari (2586) and Muslim (1623) from al-Nu’maan ibn Basheer (may Allaah be pleased with him) who said that his father brought him to the Messenger of Allaah (blessings and peace of Allaah be upon him) and said: I have given this son of mine a slave that belonged to me. The Messenger of Allaah (blessings and peace of Allaah be upon him) said: “Have you given a similar gift to all your children?” He said: No. The Messenger of Allaah (blessings and peace of Allaah be upon him) said: “Then take it back.”
According to a report narrated by al-Bukhaari (2587) [the Prophet (blessings and peace of Allaah be upon him) said: “Fear Allaah and treat your children fairly.” He said: So he went back and took back his gift. According to a report narrated by Muslim (1623): The Messenger of Allaah blessings and peace of Allaah be upon him said: “O Basheer, do you have any other children?” He said: Yes. He said: “Have you given to all of them like you have given to this one?” He said: No. He said: “Then do not ask me to bear witness for I will not bear witness to injustice.”
There is no differentiation between the mother and the father in that regard. See the answer to question number 67652. So it is haraam for her to differentiate between her sons and daughters, except with the consent of all.
It is important to know the difference between maintenance and gifts. Maintenance is according to need. For the one who needs to get married for example, the father should spend on his marriage, but he does not have to leave behind money for those who are not married; rather it is not permissible for him to do that, because this will come under the heading of gifts and not under the heading of maintenance that the son needs.
Based on that, if you are going to get married and you need some money, then your mother may give you what you need, and she is not obliged to give to the rest of your siblings the same as she gave to you, and she does not have to ask their permission.
With regard to your other sister, your mother should not give her something until she is going to get married and needs the money, at which time she may give her what she needs and no more than that.
See also the answer to question number 119655.
And Allaah knows best.