My friend took a site from a person through GPA and later got it registered to him in his favour in 2004. The person who sold the site ( two sons and one daughter). In the sale deed, only the father had signed. The property is ancestral property. All his sons and daughter were 45 years around. The site has BBMP khatha and tax paid and BBMP plan sanction recently for construction and completed his house construction. The person who sold the site died in the year 2008. In 2021, grand children are taking a route of their aunt and trying to extort money on market rates of site. He has buyers but they are asking for signatures from family tree. What is the remedy. Please advise.