When Ram passed away (i assume he died intestate i e without a Will), his property devolved on his legal heirs i e widow and children in equal proportions (i assume Ram's mother predeceased him)
So Sita could not have become absolute owner of the flat merely on basis of a registered declaration
Ideally a letter of administration for administering Ram's estate ought to have been obtained
The administrator so appointed under the above LA would then execute and register a transfer deed in favour of Sita with consent of other legal heirs of Ram
This wasn't done. So Sita had only 1/5th share in the flat and she couldn't be said to be its absolute owner to be entitled to make a Will of the same
As per Sita's Will, only her 1/5th share in the flat will go to Kush and not the entire flat
So Kush has his own 1/5th share plus 1/5th share of his mother ie 2/5th share
The balance 3/5th belong to children of Ram and Sita equally
Kush's title is not clear
Not advisable to proceed with the deal
Also merely because Kush was taking care of his mother, doesn't mean that right of other legal heirs of Ram in his flat are wiped away
Transfer by society on basis of Will and nomination in favour of Kush do not confer any valid ownership title on Kush. Its of no effect in absence of a proper registered document standing in name of Kush
Kush will have to obtain LA to administer estate of Ram and also obtain probate of his mother's Will
If either of the above grants are obtained and Kush's siblings are agreeable for transfer of the flat to his sole name, then the administrator can execute and register a transfer deed in his favour which will complete his title
A guardian will have to be appointed for the brother of Kush who is mentally unfit, by filing a guardianship petition in High Court