Can a sister have her brother's wife removed as POA?

No. Only your brother can change his POA if he is mentally competent. (Dementia does not automatically rule this out.)

Maybe a more successful approach would be to have discussions with his wife about what her objections are, and how you can overcome them. She thinks he can't deal with a lot of people at once? Arrange to visit separately. She says it wears him out? Ask her how long would a suitable visit be. Try to act like you assume she has some reasonable concerns (even if you don't believe that) and you are doing everything you can to meet those concerns.

If that doesn't work, maybe someone else here will know of any legal steps you can take to be able to visit him, perhaps away from their house.

If your brother appointed his wife as POA, you would have to prove that she is abusive to him in order to have her removed as POA. Unlikely.