In India, a registered document (including a will) can be challenged by any person whose rights, title or interest in the subject matter of the document is affected by the document which is under challenge..Therefore the general rule on who can challenge a document is that, those persons whose rights, title or interest are directly affected by the document can challenge the document.
If your mother is the absolute owner of the property, then she can Gift the property .If she done the same no body has right over such 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 donee
In such a situation no need to take signature or Consent of brother .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.
Notarised agreement cannot void the gift deed.