بسم الله الرحمن الرحيم
Your maternal grandfather has two Shariah heirs i.e. one son and one daughter. Your maternal grandmother had already died during his lifetime. Similarly, if the parents of your maternal grandfather are also not alive then all the property of your maternal grandfather shall be divided after paying those rights that are due to be paid first for example the expenses of tajhiz and takfin (funeral rites) will be paid from his leftover property. Further, if the deceased had some loan from someone it shall be repaid and then his lawful will if any shall be executed from one third of the remaining one. Then what is left shall be divided into three shares, out of which 2 shares shall go to son (your mamu) and one share shall go to daughter (your mother). It is sin on part of your mamu not to give your mother’s share. It is usurping other’s right, the holy Quran has fixed the shares of the daughters, so if one violates it he shall get severe punishment in hereafter. Thus your mamu should fear Allah and the accountability of hereafter.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.