Bank Auction Property - Sales Certificate

Dear Sir, Recently I have bought a house through Union Bank of India auction. I have registered the sales certificate in my name. Property tax and Electricity owner change is also got it changed to my name. Issue: Father, Wife & Son have mortgaged the house of 420 square yards and deposited the link documents and original sales deeds with Union Bank Of India for loan on 14-Oct-2016 Upon non repayment of loan, Union Bank of India has called on for auction of the property. I am the highest bidder and paid the bid amount. The Union Bank of India has issued and sales certificate and same has been registered in Sub registrar office. I have taken the physical procession of the property. I came to know that Bank also paid the excess amount back to borrower. Now, Son of the earlier owner has filed a Original Suit stating that I have occupied the house. As a proof he submitted the family settlement deed of part of the house ( 180 Square Yards) done on 03-Sep-2016 (One month before depositing the documents with bank) Please me help with following... 1. Does the son has right on the part of property? He is also one the borrower. 2. What are the options left for me 3. Can I include the Union Bank of India also as one of the party while responding the court summons. Thanks in advance, Khanna