How to protect him from his mom.

Sir/madam, this is about my sisters parents got my sister married in a grand way in yr 1997.nd my nephew was born in Dec,1998.then my sister nd bro-in-law got seperated (didn't actually get divorced ).this happened when my nephew was 6-7 yrs of age.then my sister went away from our house with another man who was already married nd has 2 children with his first bro in law also got married to another woman nd they have a child that left my nephew alone without parents. Nd I feel very sad to say that my sister hit her son on his male parts nd wanted him to die( at the age 7 yrs). We took good care of him from then.(actually he was at our home since he was born).my parents I.e, my nephews grandpa nd grandma brought him up the utmost love nd we also treated him as our own Brother.he is in his 12 th standard now.and will be 18 in Dec 2015. But now all of a sudden his mom came back into scene nd is disturbing him.going to his college nd all that. But he is not interested in his mom or dad.nd he wants to stay with us .we also want him. We also fear she may do any harm to him on his way to college. So pls give an advice so that she does not disturb any further . Thanking u in advance