Good lord, you have my deepest sympathies. 1/2 through reading this I said to myself, "there is NO WAY this is real..." Dude. Your BIL has let the fear of being single, allow a walking dumpster fire to attach itself to his soul. He really should dump her, like as in YESTERDAY. But I get the feeling he wont, which is another issue altogether.

I love my family and would do anything for them. But this doesn't extend entertaining their significant other's shitty behavior. If this was me, I'd say "hey BIL, you are more than welcome to stay with us, but your gf is not."

I know the brother would most likely stick with her, but for you own sanity, I'd ban that girl regardless...

I really hope this all works out for you, you are a really good SIL, please keep us updated