Hi
Supposing the gift deeds mentioned are all registered ones and there is no WILL made by the father
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1. Since gift deed is an absolute title transferring document once it is execute and accepted by the donee, it cannot be cancelled unless both doner and donee mutually decides to do .
2. In your case donor of the deed , the father died so challenging the gift deed is not easily possible
The sisters will not succeed in challenging the gift deed. Gift Deed once execute can not be cancelled( under sec.122 Transfer of Property .Act,only court can cancelled it by a declaration suit by the donor if there was any fraud and coercion happened at the time of execution,but in rare cases court would cancel the
3. The Probate is possible only if there is an existing WILL by the father. Is there any WILL , you have not mentioned about any WILL executed by the father.
4. There is no time limitation to revoke the Registered gift deed and it can be done only through a court order. and the suit of declaration to cancel the gift deed has to be filed within three years of the knowledge of the existence of the gift deed gift deed