Mother in law is 90. Diane (sister in law) has completely remodeled her whole house except for her bathroom. Which was the initial reason, With her money. Mom in law doesn't even know how much she's spent. She's so scared of her. Dr. Orderd 24/7 care but she doesn't stay there. She won't allow my husband and I there. She knows she's inherenting the house so she's fixing it up. My husband had to bring her pepto because she had an accident but he had to hid it so his sister won't see it. She's fell twice. Diane changed the locks. Mom in law doesn't have a key or the code. My husband recorded his mom saying all of this. But she's so scared of her she will not do anything or say anything. She'll side with her. But we have her recorded saying terrible things. She looks so deeply depressed. Sad. I don't know what to do.
Yes, I think sister is using Moms money to fix up a house she will inherit. You will have to prove though that Mom gave sister permission to do this. If Mom needs 24/7 care and sister is not providing it, either her doing it or hiring someone using Moms money, then I would say there is abuse of the POA. I would get a consult with an elder lawyer to see if anything can be done.
If she actually is the PoA then you should take your hard evidence of financial abuse in to be reviewed to see if you have a strong enough case.
Falling, looking depressed, and being scared of a caregiver are all things that are "normal" to dementia. It may not be abuse. If you are relying on info from your MIL, it may not be accurate at all -- also something that is "normal" to people with dementia. You can call APS but if there was any shred of cooperation left, doing this will put the final nail in the coffin. If you think she is being neglected to the point of endangering her health and well being, then call APS.
Many adult children caring for their demented parents are often accused of outrageous (and sometimes impossible) offenses. My own LO fell 2x right in her own home when a family caregiver was standing right next to her (and broke bones). She would scream "help" out the car window when I drove her places. One time the cops actually came to check out the situation.
You have to consider that your MIL is telling you nonsense and your SIL the actual caregiver has had it with your meddling and accusations. Just a thought.
Consider talking to an Elder Law attorney.
BUT if your MIL won't stand behind her accusations, there isn't any chance things will change.
Has she been a "pot stirrer" in the past and played one child against the other?
Some people do that in order to prevent their children from working together.