Gift Deed within a family (between brothers)

X (Father) has 2 sons A,B,C,D (older to younger brothers) X stayed with B and D in Mumbai in a rented room and were working in Mumbai for many years. A and C stayed back in village in ancestral property and worked there. Then owner of Mumbai plot sold the property to a builder to construct a building and X was allotted 1BHK flat in the building as part of agreement. During these period X was not in pink of his health and hence he made a power of attorney that new flat will be in name of B and D and X went back to village to stay with A and C. He died after couple of years. Its been 30 years now, B and C still live in Mumbai and C is staying alone in village (A died 10 years back). Property in Mumbai is in the name of B and D respectively. Recently, since B is a bachelor throughout his life, he made a gift deed in favor of D, executed and registered with Registrar office, Mumbai. D is still doing his moral responsibility of looking after his brother both financially and medically. Question: 1. C is in village but can he challenge the gift deed on ground that B is not medically fit and not in sound mental condition to perform gift deed (B is 72+ years)? 2. Is there any period that after that no claim can be made on gift deed. I mean, gift deed cannot be challenged by any one. 3. What if B has left a will and named C and D as a heir to his share of property? What happens to gift deed. What supersedes what? Gift Deed or Will?