Dear Ramana,
Thank you for your query! I am Aadil and I will try to answer your question.
The short answer to your query is NO. You cannot challenge the will made by a person.
The legal heirs might have had a claim on the property if the individual had died intestate or if the property in question was ancestral, as per the Hindu Succession Act of 1956. If the property was ancestral, then his successors for three generations would have had a claim towards the property and could not have therefore created a will on it, and if that happened, could be challenged in a court of law. The only other way the legal heirs can claim a property of their ancestor is if they died intestate, upon which the property would have devolved according to the Hindu Succession Act of 1956.
The self acquired property of an individual is absolutely his own, meaning he has complete rights over it including that of selling it, gifting it, or writing a will for it, as per the Hindu Succession Act, 1956. Therefore, no person can challenge the will made by a person for a property that is self acquired as that person shall have no right over it.
I hope this helps. Thank you for your time and patience!
Regards,
Aadil