Right of adopted son versus another son who is neither adopted nor natural

I am a married man of 32 years age and have a son. My family consisted of my mother and a younger brother before ky marriage in 2014. My father left house and is traceless since 2004. My dob is 23.10.1987 and my younger brother dob is 15.08.1992. Recently a dispute aroused in family when my mother along with my younger brother tried to sell land in the name of my father and grand father. Further this dispute led to the revealing of fact that I was adopted by my father and mother vide court order in a guardian case in 1989 and finally a deed of adoption (unregistered) in 1991. Furthermore I was revealed of the fact that the person whom I am considering my you ger brother is also an adopted son but he do not have any paper or anything legally. Actually he was brought by my mother without consent of my father . Although my father gave his name to him (my younger brother) but he is not legally valid and don't have right in property of my father. I want to know what I should do to prevent my mother and so called younger brother selling land and other immovable property of my father and grand father. What is his legality in my family and if my mother supports him, will he has a right in my fathers family.