One the issues here is the mom. When she has to be convinced by an alcoholic/abusing live in you should go to rountine parent teacher conferences something isn't right. Ironic the live in was no great student himself. I get the impression after a few divorces,money problems and apparently unexpected late in life child the mom is 'done'-she's had it and given up. That's why she tolerates his crap.
But for years we've never been getting a full or accurate story so you just don't know what's going in that house. Apparently the only non problem child the mom has had is living with his father.
One of the reasons I bring this up one of the tactics the live in relative likes to do is 'soften you up' with his stories of whoa for money. Guarantee you unless one of them gets a job soon he will be hitting us up for money in the name of the child. He's unofficially taken on some of the parental responsibilities or so he says but unless they get married and he adopts her as far as I am concerned the mother should be responsible for the child along with the father.
As the facts slowly comeout things slowly but steady get worse.