Adopted But Not Legally Registered, Can I Have Rights Over My Parents Property

I am Suresh (36yrs now) have been adopted (but not legally adopted and no adoption deed available) and raised by my mother's elder sister. However, all my documents and certificates have adopted my parent's name only and they don't have any immovable property. Now coming to point, My biological parents have two houses among which they wanted to write one house on my name and the other they wanted to share with my two sisters. Kindly guide how this can be done, Can gift deed/ Settlement be written by my parents for transferring one house in my name and still it will be considered as blood relation and not charged high for stamp duty and registration. I don't have any documents with my parent's name as proof. kindly suggest a simple solution in which fewer charges(Tamilnadu state) are paid and cannot be challenged later by my sisters.