Illegitimate� childrens right on property

I am the only son of my father but my mother's younger sister also starts living with my father without giving divorce to her husband and given birth to 3male. After so many years father expired then after 5years my mother also expired. Now they are claiming to make share of property in 5 people 1 share to me 3share to their son and 1to my mother's sister My father have not made any property all the land and property was purchased by my grandfather ancestral property. My mother's sister have changed her name so that her husband can't claim upon her to whom she have left and she also prepared an paper by advocate that she have 4sons including me and shown every where that my father was her husband in voter id ration card adhar card etc. Now i am helpess