Yes, it is a fact that, after 2008 as a part of the implementation of the sixth Pay Commission, Government of India has extended the benefit of family pension to widowed, divoced and unmarried daughters of the deceased Government servant. However, the daughters are eligible only after the mother's lifetime. Moreover, only one daughter, at a time is eligible for family pension. There is no question of the amount of family pension being divided among the eligible daughters.
Furthermore, the contingency that makes the daughters eligible for family pension ought to have happened during the lifetime of the deceased Government servant or his/her spouse. That is, this not a revolving gate scheme. There cannot be question of a daughter who has been divorces/widowed subsequent o the death of the Government employee or his spouse and the cessation of pension/family pension being eligible for the benfit of family pension.
Inthe instant case, the facts posted suggest that his mother has been widowed subsequent to death of the grandfather who was the Government employee and the grandmother. As such, she not entitled to receive family pension. It is also seen that the elder sister of his mother is already in receipt of the family pension subsequent to the death of the parents and she will continue to receive the benefit during her liftime. The turn of your mother would come thereafter, evn if she is otherwise eligible.