My father and my uncle own a joint DDA flat in Delhi. My father expired last month. Now my father's

My father and my uncle own a joint DDA flat in Delhi. My father expired last month. Now my father's sister says she has a Power of Attorney of the whole flat and she says she has the ownership of the complete flat . Does my mother has any rights on the property and how can we get a solution to this legal issue. We have the Conveyance Deed of DDA having my father's name and my Uncle's name. We and my Uncle have no dispute and we want  to sell the flat. Please advise on the legal issue.