All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is not valid for your mother to make a will about any of the inheritance for some of her children because it is religiously forbidden to make a will for an heir, as the Prophet said: "Allaah has given each one his right and there is no will for an heir”. [Ahmad, At-Tirmithi, Abu Daawood, An-Nasaa’i and Ibn Maajah]
You and your brother and sister are among the heirs and each one of you has his own legal share in the inheritance, so her will for you is not valid.
In case your mother makes a will, then it will not be obligatory after her death, and it can only be effective if all the heirs agree to it. In case all the heirs agree to it, then it becomes effective. However, it is a condition for the heir to agree to it to be an adult person who is in sound reasoning.
For more benefit, please refer to Fatwa 116588.
Allaah Knows best.