My brother is deceased. My niece is almost 13... Her mother has continually bounced her back and forth from mo to New Mexico. The child has serious emotional issues with her fathers death and has had very little counseling. The child pleas for me to adopt her... The mother lost custody of her first 2 boys and has been in default paying her child support many times. Also the child claims to be very scared of her mother. I recently found out the mother let a 78 yr old man watch her daughter and she was sexually assaulted... Do I have grounds to get guardianship

You would have to talk to the local Family Services agency. If the sexual assault was reported, there should be a caseworker assigned that you can talk to.