Validity of a Gift Deed

I'm planning to buy a Plot with a old house in a layout which is part of a Company housing society and the layout was duly approved by the BDA. This plot was allotted to Mr. X an employee in the said company and registered in year 1981. Subsequently the building plan was approved and a house was built in the year 1983. Mr. X has 2 daughters and 1 son. In the year 2001 the gift deed was executed in favour on his son and the complete share in the property with the house was transferred in the name of his son and the gift deed was registered at the sub-registrar office with only one witness signature. Mr. X passed away in the year 2015. Now Mr.X's son wants to sell the property. He has 2 sons, wife and a bedridden mother. He has not drawn a legally valid Family tree of Mr.X after his demise. Before the sale of this property he wants to execute a gift deed transferring his rights in the house & plot fully in favour of 1 son and his wife. Final sale of the property will be executed by his wife and his son (donees) in my favour. He is insisting on gifting the property to his son and wife in order to split Capital Gains. I would like to understand the risks associated with going ahead with this transaction with respect to: 1. As original registered gift deed is having only 1 witness signature can it be construed as invalid document? 2.Can the sisters claim stake and revoke the gift deed after 24 years of execution? Is there any law of limitation applicable? 3. Can I insist on any additional documents from the Seller to safeguard my interests?