OP so sorry for all you are going through. You sound lovely. In laws - not so much!
Sorry to derail slightly but, what does your BIL say when he wakes up on your sofa, covered in cold wet piss? Does he say, woops I did it again, as he looks at the large dark stain on his crotch? Does he say sorry but don't sit there for a while, it's a bit damp? And what does your DH say in reply? That's OK bro, mummy will look after it?
If it were me I might be tempted to write an old-fashioned letter to both SIL (please stay from me until you can be sober, civil and supportive to me in this very frightening time for me); and to BIL (please stay away from me until you can control your drinking sufficiently that you are not rotting my furniture with your piss).
I don't think either of those requests would be unreasonable. Be calm and measured - they'll think you are batshit but they already have no respect for you so heyho.
Finally, your DP really needs to step up his game. Imho his first and last priority should be to you and the DCs. If that means he tells his (definitely alcoholic) family to fuck right off, so be it.
Finally, I'm sending you loads of positive vibes and good thoughts. Sounds like you have a battle ahead of you, maybe contact all your local politicians, MP etc? Be a warrior! X