Gift means to transfer certain existing moveable or immoveable property voluntarily and without consideration by one person called the donor to another called the donee and accepted by or on behalf of the donee as held by the Supreme Court in Naramadaben Maganlal Thakker v. Pranivandas Maganlal Thakker and Ors. The execution of a registered gift deed, acceptance of the gift and delivery of the property together makes the gift complete. Thereafter, the donor is divested of his title and the donee becomes absolute owner of the property.”
Gift Deed cannot be cancelled unless it is a conditional Gift. Once gift is made by donor and accepted by donee the gift is complete. The title to the property passes on the donee . Donor cannot unilaterally cancel the gift. It has to be done only with the consent of done.
Once a gift is complete, the same cannot be rescinded. For any reason whatsoever, the subsequent conduct of a donee cannot be a ground for rescission of a valid gift. Moreover, a conditional gift only becomes complete on compliance of the conditions in the deed.
That a conditional gift with no recital of acceptance and no evidence in proof of acceptance, where possession remains with the donor as long as he is alive, does not become complete during lifetime of the donor. When a gift is incomplete and title remains with the donor the deed of gift might be cancelled.
From the above aspects, you should check the copy of new deed executed by your mother. And file a case for setting aside the new gift deed executed in favour of your sister.