I am 46 a women whom I call mother living with us how make this legal

I am a 46 years old man .A lady aged 70, deserted by her husband, has been living with my family since 1998.She introduces me as her Son to the society and I have also given her the place of Mother. She has two married daughters with whom also she does not have any relationship either.No one ever cares for her.I (now with my wife and daughter) have been looking after her and serving her like my own mother.We are from different castes.Can this relationship have any legal strength? Can I make any legal claim to her property? It is her wish too. She wants to give me all that she has.She has not legally divorced her husband.Her husband and daughters have not contacted her for more than 14 years though 2 of them live in the same city.But I know they will come if anything happen to Maa and will blame me for that.Will I be rewarded for all my services to her like that?I want to know if there is any rule which accepts this relationship as legal and give me the rights of a son?