1. Title of property is decided based on registered title deeds and not on oral declaration,
2. For executing and registering the gift deed for transferring his flat, you father shall have to have a title deed evidencing that he said flat stands in his name,
3. Your father and all uncles & Unties shall have to register a settlement/partition deed wherein it should be mentioned that the individual shares have already been mutually partitioned/settled which is being registered now,
4. Even then conservative buyers may hesitate to buy the flat from your sister since the gift deed in favour of your sister was registered before your father had registered the said flat in his name,
5. The ideal course of action is that your father and Uncles & Unties first should register a partition deed after which your father should register the gift deed in favour of your sister