Distribution of property with Father's sister.

Good day to you gentlemen, I have a query and I'd appreciate your suggestions on what I should do. My father has four brothers (him included) and three sisters, one of the them passed away, now he has two sisters. All the brothers had their fair share from the property, were satisfied. They left and now living on their own. Now the sisters, one of them lives in abroad and the other one lives with us in the house which is next to ours and belongs to my father (it's on my fathers name). She had initially agreed to leave the house if we were to pay her 15 lakhs which seemed feasible. We all agreed to it. We have her signatures on paper which might serve as an evidence. With that 15 lakhs, my father bought her a new house, as per the deal. Now she's been demanding 30 lakhs (15 more than initially agreed) or else she's won't vacate the house. All the members of the society are with us. Even my Uncles (Father's brothers). I'd like to know what are my legal options as the house in which my Aunty (Father's sister) is living in, belongs to my Father. On the paper and even on the electricity bill. All the taxes are also paid by my father.