Hi. I have primary physical custody of my niece. Her brother lives with his grandma and gram is seeking full custody- with adoption, terminating mom's rights. I have a signed agreement with mom and birth certificate dad- not the biological dad- for shared legal custody but now that she's losing rights to her son, I was wondering if I can have her rights terminated for my niece too? She doesn't pay child support for her, never comes around anyhow and only harassed us when she feels like it.. before anyone thinks I'm doing this for no good reason...

Absolutely. Each case is individualized by incident, meaning that if there is an incident with one of three children that child will have a case before the court, the other two would not. However, if there is a charge of neglect/abuse, the family services department will investigate, then can and typically will bring a case against you on the other children (the cases are usually joined at that point) if there is a serious situation that endangers the children.