1. Since the original allottee has gifted the MUDA allotted Site to her Son and if the lease period is already over, then in the Seller's hands it is his self-acquired property.
2. Even though the mother is having 3 Sons and 2 Daughters, the legal heirs have no right over their mother's self acquired property during their mother's lifetime.
3. It is the prerogative of the owner of the self acquired property to take any decision regarding the property and in the instant case she has gifted the property to her son. I hope it is adequately stamped registered gift deed.
4. Executing the gift deed in favour of her son is her exclusive right and other legal heirs cannot question her absolute right in the matter.
5. Moreover, as narrated by you, the mother is also signing as consenting witness.