Oooh! This gets into some really sticky stuff. You DO need to find a way to make sure they don't rob your father of stuff that is his or jointly owned by your father and step-mother.
This is where having everything spelled out in wills is a good idea. But if that hasn't been done, is there a trust officer, an attorney or office of the court who can we called on?
A pre-scheduled time for stuff to be taken is also a good idea, probably with someone present who can act as an arbitrator or otherwise oversee the "gifting."
Ideally you or one of your sibs can be present at that time to represent your father's side of the equation.
My prayers are with you. Also, we'd had at least one article from member Julie Hall about this on NABBW.com in the member articles section. She is known as "The Estate Lady," and she got into the estate business after one of her elderly clients was robbed of all her stuff by friends and family -- except that no one could be charged because she knowingly let them take her "stuff."
Here is a link to her articles: NABBW Member Julie Hall, "The Estate Lady"
Hope this helps!