Dealing with a sister-in-law that moved in to help take care of mom. How to handle all of this?

DH and I have lived with his mom since 2003 when she had her stroke. The other siblings didn't want to deal. So now, 13 yrs later, the sister decided she hates her job, and doesn't want to spend her vacations here with her mom, so decides to move in with us. She quit her job and didn't tell her mom - just moved in. Didn't even get her a Christmas present either. Her mom is 87 and has dementia, but not to the point where she doesn't remember our names - it's just starting to head down that path. Anyway - she's lived by herself and never been married (She's 55). We have let her take over the care for her mom since we have been doing it for a long while - but we still participate in some things. The problem is that she complains about how we did it before she was there - how we had separate trees (we live upstairs so we had a tree up there) she had one downstairs. How we separated food, how she hated the microwave (she couldn't run it, and it died, but how she doesn't want to buy a new one. And how things should be divided into 3rds (her mom's mother's day dinner for example - Hubby and I were one, and she paid the other half). I dread coming home, and hubby knows sister is loony, but won't confront her. I'd love to move, but hubby says we can't afford it - and we'd probably lose 1/3rd of the house we are supposed to get that we could start our new life with. How would you guys handle all of this?