If I were you I would honor my agreement and leave and I would take the niece with me. Like Anvil said, once we were settle in new 'digs', I would get her a tutor, I would give her all the attention I could, I would sit near by when she did her homework and stress that i will answer any question she has, to the best of my ability, both about her homework and about her life.
She is old enough to go to Alanon or check with her school counselor to see if there are any Alateen meetings that might meet at her school, otherwise I would research Alateen in the area and introduce her to Alateen where she will find peers that also have a father, sibling, mother, grand parent(s) that are also practicing their addiction.
Please know that we are walking with you in spirit and we are here for you.
Love and hugs,