بسم الله الرحمن الرحيم
(Fatwa: 694/1076/N=9/1434)
In the case mentioned above when your Nana is no longer mentally strong then your Mamu is his wali (guardian) as per the Shariah.
قال فى الدر (مع الرد 9: 214، 215 ط مكتبة زكريا ديوبند): و عندهما يحجر على الحر بالسفه والغفلة ، وبه أي بقولهما يفتى صيانة لما له ، وعلى قولهما المفتى به فيكون في أحكامه كصغير اهـ وانظر الرد أيضاً .
Hence in the question mentioned above the land of your Nana which your Mamu has sold if it was useful and good then it is alright without any doubt. But the money shall not be distributed among the children of your Nana and wife right now as so far it has not turned inheritance rather it is only the property of your Nana. However, it shall become inheritance after his death and shall be divided among all his legal heirs accordingly.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband