Can family members protest the sale of our parents' home, by a sibling who recently obtained POA from an Alzheimer's parent?

Probably, by getting legal assistance on how to invalidate the POA, assuming that the parent was in fact unable to understand what he or she was granting. And that depends on the seriousness/stage of the ALZ and at what cognitive level the parent was functioning.

Is the proxy under the POA selling the home to get funds to keep your parent in a facility? What are the other facts on this issue?

If the parents are married and the home is the marital residence, the POA could not legally sale it in most states without the consent of the other parent - even if the house was titled in only one parent's name. I don't think even Medicaid pushes the non-ill spouse out of the home. If I remember correctly, Medicaid places a lean on the home and gets their money when the house sales after the other spouse moves or dies.

If only the ALZ patient was living in the home and it was titled in only the patient's name, then the POA could sale it, but you can still challenge the validity of the POA. You need to see a good elder law attorney. Filing for Guardianship might be the best way to challenge the POA.

Is the other parent one of family members not in favor of the house sale? If she is living there the POA could not sell without her agreement. In fact, I doubt he could even find a real estate agent to list it without the other spouse's signature.

If they both want to sell, then the POA is responsible for making that happen. But he can't impose it against their wishes.

H'm.

As has been explained, if the home is owned by both parents, then the son will not be able to sell it using only his POA on behalf of one of them.

But you're asking what the siblings should do about the situation. And I would first like to ask: what are the siblings' reasons for disagreeing with the sale of the house? And what are the siblings' proposals for providing care for both parents?

It's a question of who is doing the actual work, you see.

Juanderful, you've gotten good advice from all prior responders, given our limited knowledge of the situation. If, after answering the questions asked, it is clear that your interest in your parents' house coincides with what is in their best interest and assuming that the house title situation will actually allow the POA to sell it, then, as GardenArtsist suggested, you could challenge the legality of the POA, but that may be a difficult (i.e. expensive) case to win or not, depending on details of your dad's dementia diagnosis. If a POA challenge looks to be difficult, then, as TNtechie suggested, filing for guardianship may be the best solution.  Guardianship/conservatorship, if granted, voids any and all POAs.  It will also be expensive, but perhaps no more so than a difficult POA challenge.