Hi, Maryann,
First of all--hey, if you can't ask questions on a Board specifically designed for you to ask questions--where CAN you!

There is no such thing as "too many questions", here. Your specific problem will certainly be another caregiver's problem at some point! Although each individual situation is somewhat different--a definite pattern seems to emerge in most dementia care cases. So--keep writing and asking!
One of the best suggestions I ever got (way before this Board was "born") was to hire an eldercare attorney. Not being one to run off to lawyers (I had an unspeakable ex-brother-in-law lawyer, who always seemed a tad too "eager" to get more info from my younger sister about our family's estate), I, nevertheless, DID get one whose field was eldercare.
He covered every aspect of "what if"--the legal and financial implications, what problems could possibly arise from my Mom's will, how to cover every potential problem that might come up.....including just what my brother-in-law might be "checking out" from past documents my sister might have let him read. We had two meetings with him....first, to discuss our specific situation...and the second, after he'd evaluated all the nuances of any legal documents my mother had filed.
I would never have thought of all the things he brought up and took care of. He followed our case until our house was sold.....and, after my Mom died, until the will was completely settled and the estate stuff dealt with.
It was the best $800 I ever spent. He was always available for our many phone calls with our many questions! It gave my twin sister and I such peace of mind!
Re. your brother and sister-in-law moving into your Mom's house....I'm confused. When you say your Mom can live in their vacant apt in their house...which house do you mean? I assume, if you say "you can take your Mom to her house on weekends"....you are saying that your Mom would NOT live with them? Please forgive me for my very own senior moment. But I'm just not certain I "get" this, correctly! LOL!
I have to say, though, that my instinctive
emotional reaction, without even knowing all the facts, would be so similar to your's--particularly if you mother doesn't "love" your sister-in-law. My own mother never got to the stage where she would have been oblivious enough to her obnoxious son-in-law moving in! LOL!
Your instincts about just how your sister-in-law would deal with the difficulties of your mother's increasing dementia--just how "loving" she would be--are certainly worth serious consideration!
Lynn : )