Have you ever considered that maybe your friend shares his family's most noticable trait, namely being manipulative?
It seems he got you somewhat distressed about his situation, even asking for help on forums on his behalf,
while he is not actually doing anything that you have suggested to him before, except saying 'I might try that'. Correct me if I'm wrong.
Now I am not saying that his situation is healthy, at all.
It looks like there's a bunch of people in that house with a distressing lack of empathy, or ability to take responsibility for their actions, or inactions.
That can and will only degenerate into a black, depressing, downward spiral, where everyone reinforces the other's behaviour.
Perhaps the very best thing you can do, is instead of giving your friend well meant advice, that he probably will not follow up on anyways,
tell him, how his constant complaining, combined with his apparent lack of initiative to really do something with whatever advice you give him, freaks you out, and makes you feel bad, and that that troubles you.
It can be an eyeopener for some people (especially borderline manipulative personalities) to hear someone else speak out loud, how their behaviour makes you feel.
Tell him you do not want to hear anymore of his complaints, until he can show that he actually is taking action to solve the issue.
Best thing for him, in my opinion, is to kick out his niece with a month's notice, to at least give her a chance to find something, and strongly encourage his mother to find her own place as well.
Cause as some people said before, this current situation can only escalate.
If you do manage to get him to take action, it's a good first step in learning him to reflect on the consequences of his own actions and inactions. He'll thank you for it later.