Can us kids get POA overturned on our step dad for our mom? She has dementia and he put her in assisted living against her wishes and ours.

You can see an attorney to seek guardianship of mom. If you were to succeed in court, then how would she be taken care of?

He has the legal authority to make care decisions on her behalf if she is cognitively or physically incapacitated or beyond the care that he can provide for her. If he's the only caregiver for her perhaps he is burnt out and tired. How old is your mom? I'm so sorry for the distressing division in the family. I agree that if you suspect there is some sort of controlling/abuse you should consult an attorney.

He has the POA and he has a right to put her in an AL. I would assume he was her caregiver.

Not allowing you to see her is another thing. IMO this is a power thing. This is not what POA means. I doubt Moms wishes are her kids can't see her. I would consult with a lawyer.