1. your father was co-owner of the flat having 50% undivided share in it
2. after his demise his share will devolve on his legal heirs as per Hindu Succession Act to his widow, mother and children
3. i assume when your father died, his mother was not alive
4. since your mother was living when your father died, his 50% share in the flat will go to your mother and you equally
5. consequently your share will increase from 50% to 75%
6. your mother inherited 25% share in the flat from your father
7. since she also died intestate, her 25% share will go to her legal heirs as per s.15 of HSA who are her parents, husband and children
8. i assume your mother's parents died before her. So her 25%share will devolve on you
9. consequently your share will increase from 75% to 100%
10. however in order to transfer the 25% share from your father to you and the 25% share of your mother to you, you will have to obtain letter of administration for both
11. the administrator under the above 2 LAs will then execute and register transfer deeds in your favour which will complete your legal title to the flat as 100% owner
12. since you do not have a title document for the balance 50% share in the flat, even if you transfer that to your daughter, the defect will not be cured and the objection of the buyer will still remain, until the LA procedure is done