Dear Queriest,
You say your brother has got the property by way of Registered Gift Deed. Can you prove in a Court of Law that the Gift Deed was made by your father when he was not of sound mind? It is a Registered Gift Deed and while Registering it, the Registrar representing the Government has satisfied himself that the Gift was made by your father properly and as per the procedure and hence registered the Deed. Otherwise, Registrar would have refused to Register the Deed. Now the key may be the Witnesses to the Gift Deed. Will they come and under oath say that your father signed when his senses were not with him and thereby admit that they added their Witness when they know that his capacity to sign the document was not there?
Further, from when your father was Schizophrenic and when the Gift Deed was Registered. As per the law, even a lunatic can enter into a contract and it will be valid provided, he entered into a contract during his spells of normal behavior. Therefore, if your file a case, you are free to do so. But the issue becomes a Medico-Legal case and onus of proving that your father was not of sound mind at the time of Registration, as against solid and strong evidence of Registered Document. I am not trying to discourage you but trying to inform the pitfalls. Fingerprints on wrappers of tablets can be only circumstantial evidence and may not be accepted as the primary evidence as it is difficult to prove that the tablets in those wrappers were in fact given to your father.
Depending upon the value of the property, you may take an appropriate decision to file a case or not.