Ancestral property ongole

Hi my grand father brought a house in my home town(not sure if he brought after selling Ancestral property or from his own pocket) with his name, my name and my brother name on sale deed. he brought that property on 1961. As my mother is widow we don't have any other income source from my childhood. she sold all agriculture lands we have and run my that time my grand father think that my mother can sell this house also for money so he kept his name, my brother name and my name on sale deed. we sold some part of the property and used those funds(dowry )my sisters marriage(1980's). she took divorce(after 5 yrs) and left her 4 children with first husband. she never claimed anything till now. we have all current bills, property bills on my and my brother name for past 20 Yrs and we are staying here from past 20 Yrs. now my sister threatening us that she has share on this property and do partition. Not sure how to proceed on this please guide us to get the property back. what and all chances are there that we can stop her getting share on this. I am retired and staying in the same property. we don't have any other home to stay. Please help us.