It might be a good idea to apply for government housing (if you haven't already). It's not a short term solution but something you can look towards in the near future.

I suspect your brother in law is still living there because they're both financially contributing to the household, relieving some stress on your mom.

As for his snoring, maybe your sister can help address that issue. It could be a sleep apnea issue or a deviated septum, with a medical solution so everyone can get a good sleep.

Your sister's window.. if it's too hot to sleep with the door shut, it can't be too cold to sleep with the window open a slight. Talk with her again about it and see if you can all compromise.