Land sold but registration not happened only agreement is there

Muslim -Property Case - Hi, My Grandmother on her name had 2 acres of agriculture land and sold it to 30 years back for 10000 rs. My dad, and grandparents signed in the sale agreement . SALE REGISTRATION not happend til now .My dad also have 2younger sisters who were below 15 age at that time. few years back that buyer(no kids) died and now his brothers son-in-law asking my dad to Register the land on his name. Now the property worth is 30 lakhs. Now my query is it legal that we should do the registration on his name,? do we still possess ownership on it ? can we demand some money to do registration ? do my aunts also sign the registraion? what happens if my dad and grandmother alone sign the registrtration ? in future is there a scope for my aunts to make the registration invalid ? if so , will my dad get into trouble in that ? Kindly help me to move forward.