Not necessary that sons should sign as witness to property owned by parents. You can even have independent witnesses sign sale deed.
Sir, I'll provide you certain legal suggestions, but for that i need to ask certain questions from you. For any further assistance, feel free to contact me on my e-mail "firstname.lastname@example.org"
Shubham Pabbi & Associates
To give opinion, it is necessary to know whether the plot of land upon which the house was constructed was ancestral or self acquired property of the Sellers. If you wish to take my legal opinion professionally, then can whatsapp at 9830019661 or can EMAIL email@example.com .....Sinjari Bandyopadhyaya, Advocate of Calcutta High Court