We had a joint property of 27 acres in Krishna District, Andhra Pradesh. Its document was in my grandfather's name. Almost twenty two years ago, our family had an understanding that my grandfather gets 6 acres, my father's brother gets 8 acres and my father gets 8 acres. The remaining 5 acres was to be sold to pay off the family debts. However, my father took that 5 acres on condition that he will pay the debts by himself and he paid them off. In 2009, just six months after my grandfather's passing away, we sold his 6 acres plus my uncle's eight acres (for his daughter's marriage). As our share, we got the money for grandfather's 3 acres. Now 13 acres are remaining. In that, 3 acres is in my mother's name, so that is ours. In the remaining 10 acres, my father and uncle have equal right on five acres each. So, as per the previous understanding within our family that my father should get extra five acres (for paying the joint family debt), my uncle gave the 5 acres to my father through "Trasfer of Right" registration. It is not a sale agreement or registration. During the registration, his only daughter, i.e, my sister did not sign though she is a major at that time. She acted only as a witness and gave her witness signature.
Now, six years after all this, my father wants to sell a part of our thirteen acres. It is in two partitions: 10 acres and 3 acres which, as already mentioned, is in my mother's name and has a seperate pass book. My father is afraid that my sister may claim right on 2.5 acres of the 10 acres. So, he is willing to sell only acre 7.5 or less in that. The 27 acres that my grandfather originally owned is what he purchased from another party. It is not inherited by him from his parents. So, as the previous sale happened six years ago, will my sister win if she goes to the court of law or will the court accept the case, atleast? How genuine is our document? And can we sell more than 7.5 acres or the whole 10 acre bit?