The step daughter of grandmother will get share from the property of your grandfather, not from your grandmother’s. If grandfather gave her some property in his lifetime, it will be termed as gift and it will not terminate her inheritance right. In the case mentioned above, all his property, after paying all due rights preceding inheritance, shall be divided in 96 shares, from which the son will get 50 shares and the real daughter of grandmother with get 25 shares, while step daughter will receive 21 shares. Those daughters who passed away in the life of grandfather and grandmother shall be deprived.
Son: 50 shares
Daughter: 25 shares
Step daughter: 21 shares
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband