First, having gone thru this several times, I will tell you there are always
members of the family in denial. Are you home a lot on the weekends, or
do you leave her alone for several hours. If you do leave her, I think it is
in the best interest if you can get a companion for her. Unfortunately,
Medicare does not cover this.
Unfortunately, it sounds like your SIL will get some help when something
happens to make her realize the severity of the situation. Is she safe
at your SIL house? Are there young children in the home.
You will have to check and see if Medicare will cover an aide. Does your
MIL take care of her personal needs; showering, toileting, etc. If she cannot
take care of herself, Medicare might cover an aide for a few hours a few
days a week.
I think you need to sit down with your husband and his sister. Sounds like
you are me! The level headed one. Good luck.