There is a limitation period for every transaction in litigation and a registered document can only be canceled through a competent court by filing a declaration suit. Those who are not in possession of the property have to file a declaration suit and pay a court fee on the market value of the property and file such declaration suit within a limitation period. In case of fraud, the opponents have to file the suit within 3 years from the date of knowledge.
Hindu law is having a double edge and complicated to interpret without knowing the facts. If the property is not divided in metes and bounds in an undivided family, any of the co-sharer may have the property in his name as Trustee. That means if the family is undivided, the burden of proof that the family is undivided and the property was acquired during such period is on your father. The presumption is that a Hindu family is undivided when there is no such evidence of partition. That means as a co-sharer your father is having such property in his name and may not amount to exclusive possession. The contents of such title deed is important. The chronological events are more relevant.
Finally, the opponents have to file a declaration suit against you which may take decades and defend when such suit is filed and do not be disturbed if your father has obtained the property in a legal and justified manner.