My mother currently has custody of my niece since she was 3 months old, she is now 2. My brother, the father of my niece is deceased and the mother is in jail. My mom is willing to sign her rights over to me. Who would I need to contact to get custody of her?

Your mother can't sign the rights of anyone over to anyone else... people aren't used cars.

You would HAVE to go through the courts.

Smoothy is correct. You need to go to the courts . There is no other way.

You file a petition with your local Family Court to obtain custody. If your mother is agreeable to this then there should be no problem. Its possible, however, that the bio mother will have to be consulted. Especially if she had agreed to allowing your mother custody.