Depending on her age/ bills/debt. I don't see that being too bad. When I was 22 making 30 grand a year, with only a car payment I paid 1200 a month and had plenty extra to save and spend.
I'm on the app, but I'm making that face with the giant eyes and straight mouth. Holy crap! That is a huge difference. Why in the world is she quitting!?
My first mortgage was $1,100 a month, and association fees brought that up to about $1,300. I made $45k for my first year in the house. I did rent a room to a friend for $600, but I could have made it work without her rent if I absolutely had to. That said, I have a feeling your SIL doesn't quite grasp how tight money is going to be if she's going from a six figure income to $45k with rent that high. Had I done it without a roommate, I would have had virtually no money for eating out, going to the movies, getting my hair done at a decent salon, etc.