...slightly less suicidal than I have been for the last few months. I had an honest chat to my SIL and laid my cards totally out on the table, told her where I felt able to compromise and what was staying. She's going to talk to my BIL about it and see what he says. If he's willing to compromise too then all three of us will be happy.
I'm still very suicidal, but not every breath is agony any more and I'm no longer at immediate risk of putting my plan into action.