I wont have time to look back here, but I'll just say a couple of things:
It sounds like steve/michelle DID trust Janet, even if they no longer do. They HAVE been supportive -- they let her move in and she only pays for food and cleans up after her dog. Seems like they've shown a lot of support and mercy given the situation.
the problem has come up and gone on for a while now. They've been supporting her and are still doing so.
I really like these questions actually, because people/relationships are extremely important, and this is the heart of the issue. M/S don't want to upset Janet -- I think they DO want to support her. I'm making the assumption that their rules for her are to help the relationship between M and S, and between M/S and Janet. What's happening is the opposite of all of that, because Janet, however much she is hurting, is looking for happiness in the wrong place or at least the wrong way: at the expense (relationship wise) of those who are helping her.
What can be done? I find it easy to let things go, but harder to confront out of love. I think that needs to happen, but in the right way... very very carefully! Janet may hate them for it, but if they had any relationship to begin with, she'll eventually see that M/S wanted what's best for her and them. And even if she doesn't ever come around, M/S can sleep at night knowing they did everything they could to help her with sincerity.
(sorry for quoting you sj. I just liked what you had to say and wanted to respond -- wasn't picking on you!