Daze, I think the support you need must come from real life, and not here. You and your family sound like you need lots of different kinds of support. You can't do anything about your BIL, he's made his bed, he'll have to figure out how to get out of it, or sleep or whatever.
You need to help your SIL get help. If she is that bad, and acting suicidal "she mixes the pill's with the booze". I agree that taking her to a bar is not at all helpful. If she won't change her ways, then you shouldn't be supporting them because it's going to end badly for the niece.
I applaud you for looking out for your niece, and if things keep going this way, I suspect you're going to be her family.
As far as your kiddos, there's other threads that can help on Mumsnet, but I would really suggest that you run your responses through a word processing software (like word) before posting, to streamline the process and to allow you to read it more than once. This is what I do, and I am not struggling with anything like what you deal with, but I still struggle to make sense!
Talk to your husband about what services may be in place for your SIL, because really, he's going to make his niece's life a lot worse by not helping out her parents.
And perhaps the two of you need to discuss takingyour neice o full time, let the parents be the people they want to be, but try to save the child!