cancellation of gift settlement deed

Dear Sir/Madam I am a Hindu male aged 40 years from Chennai. My grandmother had purchased a property in 1956. She died in 1978 without leaving a will. After which my father and his four brothers (total five of them) had partitioned the property into 5 portions through a registered partition deed in 1988. All my four uncles have sold of their properties. My father constructed a house in his portion and is still alive and has given his property to me through an unconditional settlement deed in 2009, all municipality and electricity records have been changed into my name and taxes, bills are being paid in my name. Now my sisters are threatening to file a partition suit case against my dad. My questions are: 1. Is my grandmother’s property ancestral property? 2. Once the registered partition has taken place between him & his brothers does the partitioned property become self acquired property? 3. Is the registered settlement deed made by my father to me in 2009 valid? 4. Can my sisters file a partition suit against my father? Advance thanks for your assistance. Regards