Your not screwed ! Your just under ridiculous stress due to the SIL intruding. Get a day/date that she will be gone, I think it will help if you know when she's leaving, Seems if you know when X will happen it makes it easier to cope with the days between or at least that helps me.

When I reach my "boil over rage fest" That's usually when the scream into a pillow starts.. So... somehow some way find a release for the build up... scream, rant, blast music and do an angry dance, any other thing that doesn't cause you pain.

Keep posting here !

Hugs