Sister and Brother-in-law claims my personal property.

17 years back I was alloted a 1000 sq. Ft flat in Mumbai suburban area by chief ministers quota of 10%. in my name. I started living with my sister and brother-in-law and their 2 children. Since my brother-in-law had helped me financially to buy that flat, which rates were 10 times cheaper compared to market rates. After 4 years of stay my sister and brother-in-law signed a ir-revocable power of attorney with me promising me to help financially to set my own business. The ir-revocable power of attorney was drafted on a Rs.100 stamp paper and was only notarised but it was not registered with the registrar. Till date it is not registered. Now since 13 years have passed they have not fulfilled their promise to help me financially and I m living in a rented flat since last 13 years. My sister and brother-in-law already own a flat in Mumbai in their name, but currently they dont stay there. Now they are forcing me to sign a gift deed in the name of their elder son because according to their laywer the ir-revocable power of Attorney which was signed by me is of no use since it is not registered. I m helpless since I dont have the original copy of sale agreement and share certificate. But I have a xerox copy of letter of allotment of the flat from URBAN LAND CEILING (ULC) Department, government of Maharashtra. They have been paying the society maintanance charges since last 17 years through cheque. Can I claim my property back ? What are the legal proceedings ? Pls help.