POA

Call APS for wellness check, senior at risk. Let them know what you have told us, that your stepmother may be ill with alzheimers and your Dad is helpless and can't speak, and you are locked out. Tell them this is an emergency.

When you were last at their house, how did it look? Was it filthy? Was there unopened mail? Garbage piling up? Pills on the table? Rotting food about? Unmowed lawn? Because if not, then I'm not sure calling APS will do anything unless the visiting social worker has a conversation with your SMIL and sees signs that she's cognitively impaired or your Dad is suffering from neglect. BUT even so — if your Dad seems competent, then APS probably will leave them alone.

Guardianship will require that your prove your SMIL is an incapable caregiver. It is very expensive. I would consult with an elder law attorney to see exactly what options, if any, you have right now. You may just have to wait for a full-on trainwreck. Your situation is actually very common, if you search on this forum you will see how common a problem it is. I wish you success in getting them both the help they may need, now or in the future.

I recommend make a call to Elder Services or CPS-type agency. She's isolating him from his family. They should come to find out why.

They can make a determination what's in his best interest, and perhaps they can evaluate her (I have no idea how in-depth).

The only thing I can think of is that you're going to have to file with the courts for guardianship over your dad.
I believe that can be a bit costly, but if you're truly concerned about your dad it will be worth it.
Or I guess you can call APS and report a vulnerable adult and they will come out and do an assessment on the situation.
I do hope and pray that everything works out as it should.