Letting Sally under the LW's roof is a terrible idea -- there is absolutely no way that any deadlines or "house rules" can effectively be *enforced* once she's in . . . unless you go the legal-eviction route, which could cause a breakup with Sally's mother.

For the sake of LW's sanity and relationship, Sally cannot move in.

And, yeah, the advice in the second letter is terrible. Something is going on with those kids, and they need a mental-health intervention rather than "discipline."