Are aunts who do not inquire about their mother (the grandmother) entitled to inherit her saved money after her death, and is it permissible for my mother to donate this money as a perpetual charity on behalf of the grandmother and my mother after her death?

The severance of ties between your mother's two maternal aunts and their mother (your grandmother) is an act of disobedience (ʿUqūq) and a grave sin for which Allah hastens the punishment in this world. However, this does not justify withholding an heir's share of inheritance. You are not obligated to spend on your grandmother if her pension is sufficient for her needs. What you spent with the intention of donating cannot be reclaimed. However, if the intention was to be reimbursed from the estate, then there is no harm in taking back what you spent. Your mother's act of giving continuous charity (Ṣadaqah Jāriyah) on behalf of your grandmother is a good deed, but this cannot be taken from your aunts' share of the inheritance without their consent.