Custody of my cousin brother.

I have a cousin brother who is my elder uncle's son. He is mentally challenged but can do his regular chores. He had his mother who passed away in the year 2010 after which his elder sister left him unatained because of which I brought him to my house and now for six years he stays with us. Now the our elder sister wants to take him away with her after this six years but me and my family believe that it is because of a property that they jointly own after the death of their mother. His father passed away in the year 2013 after which she came to take him away but my family members protested against this and she couldn't get this done. Now the thing is that I am a bit worried about my current stand in this case and my elder cousin sister has been repeatedly bothering me and my family along with my brother for last couple of months and even in front of the police officers the matter was taken forward and there my brother stated that he wants to stay with us. If you could help me with a proper procedure and way by which I can look into saving my brothers concern then it would be really helpful. It has also came to my knowledge that she has filled for a custody case in the court.