Is deceased sister's son can claim in mother's father property

Hi, We are Muslim family. My grandfather had one elder brother who had two daughter. After death of elder brother, my grandfather got married with brother's wife and blessed with one daughter and one son. As he had two step daughters from elder brother, these two step daughter were the owner of half of the all property. So my grandfather took signature on stamp paper stating that they are giving half of the property share to my grandfather with their consent and they do not have any issue. Now one of that step daughter has been died and one is alive. I have below queries: 1. Can deceased daughter's husband/children still claim shares in grand father's(Mother's Father) property as currently my father is the managing all the property after my grandfather death? 2. Can alive step daughter's children claim share in grand father (mother's father) property. 3. Can my father real sister claim shares in entire property or only half share excluding step sister's half share's. 4. Is Hindu Act is applicable for all community or for Muslim family there is separate rules for it. Looking for your expert advice on below queries. Regards, Shakeel Ahmed