Are we obliged to accept our mother's waiver of her share in the inheritance from her two sisters in favor of their daughters, knowing that we are in dire need of money?

It is permissible for a mother to donate whatever she wishes from her wealth to her sisters' daughters, whether it is from an inheritance or private funds. She is not obligated to give anything to her children or grandchildren. The general principle is that a person has the freedom to dispose of their wealth, and they cannot be compelled to do something they do not wish, nor can they be prevented from doing something they desire, except for the purpose of counsel. As for children, a father must be just in his gifts among them, while justice is not required among other relatives or strangers, and there is no obligation to give children the same amount that is given to these others. Therefore, there is no harm in the mother giving her inheritance from her sisters to their daughters, and you have no right to object to her actions.