بسم الله الرحمن الرحيم
As per the above mentioned case, since Mahmood was alive at the time of his father Hussain Shah’s death hence he shall also be the Shariah heir of his parents. If at the time of Hussain Shah’s death none among his parents was alive likewise at the time of his wife’s death none among her parents was alive then the property of both of them shall be divided into three shares, out of the total 2 shares shall go to Mahmood while the daughter Zaibun Nisa shall get 1 share. In case Mahmood is missing in such a way that it is not known whether he is alive or dead then his case shall be produced in local Shariah Panchayat. If the member of the Shariah Panchayat after much investigation come to a conclusion that the missing one has died then his property shall be given to his son Mohsin, as he shall alone be the heir of all the property of Mahmood.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.