Did the mother act justly among her children and not wrong the two daughters by allocating an apartment to each son, while the two daughters shared one apartment for which they paid approximately half its price, and their younger brother will receive an older apartment whose value exceeds theirs?

A mother's will to her children regarding her ownership after her death is not valid, and it is not obligatory to implement it. The heirs have the right to return it to the estate and divide it according to Islamic law. As for her gift, justice in it is to give the female like the male or half of it, according to two opinions. It appears that she did not give you anything at all, and this is not just. She must fear Allah and be just among her children, for injustice is forbidden by Islamic law and breeds resentment.