Personal story.... - buckup
...put the child first. The need for that young girl to find a loving, safe place when her mom can't deal with her is critical. She may get her only positive male image from you.
A friend of my daughter's was in exactly such a situation. Both played AAU basketball at a pretty competitive level. Maya's mom would drop her off at weekend tournaments for the first game and leave, knowing someone would bring her home. Maya would have no money, someone would buy her lunch. We were totally being used, but you need to think of the child.
Twice Maya lived with us for weeks at a time. Never formally moving in, just landing at our house each night and sleeping over. Things had gotten spooky at her house. however, her manners would be like Mogli's from the Jungle Book and we told her she was always welcome, but needed to clean up after herself. She got mad and left, but, came back.
When she was with us, she had to go to church with us. She decided to work a bible camp summer of her junior year in high school. She met a terrific Christian kid from a super stable family. They were married in the summer of 2014, after college graduation. My wife and I were invited to the wedding, where Maya introduced us to everyone as "the closest she ever had to a real family". We had never discussed such a thing and I was so humbled by it, I had to choke back some emotion. She had been a pain in the ... but deep inside, she got it.
Maya became a parent in August. We were the first non-relatives to meet the little fella. Its not quite second daughter status, but it is an appreciation that wont go away.
When you sow the seeds of kindness and compassion, sometimes they land on rocky ground and wither, leaving you feeling used. But when they take root, it is glorious. But, remember, when someone takes advantage of your kindness, that's a reflection on them, not you, so just let it go.
The little girl is lucky to have someone who cares about her to the point they'll pick up the slack when her mom doesn't. You'll never regret the time invested.
The height of confidence is standing up in a hammock.