Once all the legal heirs have mutually consented to execute a registered gift deed to one among them and the necessary conveyance formalities have been completed legally, the said gift deed is very much valid and irrevocable. The donee, i.e., your mother becomes the absolute owner of the property gifted to her once she acted upon the gift deed by taking possession and getting all the records transferred on her name. In such a circumstance, even the donors themselves do not have a right to revoke the gift deed hence there is no reason for their next generation heirs to claim any right in the property. This is not an ancestral property carrying minor interest at the time of execution of gift deed, hence need not worry about it. He is not entitled to any share or right in the property.