Is it the law that all siblings be included when choosing a POA? My mom wasn't in her right state of mind when she chose my brother.

If she signed for him to be one she can also revolk it. I have heard this before about hospital getting involved with POA. I can't see where this should be allowed. She could have signed HIPPA paperwork but a POA would need witnesses and a notary. I would question the SW on this and ask for proof. If Mom has been diagnosed with Dementia, she is not able to make an informed decision.

No, siblings do not need to be made aware who a parent has assigned POA to. If she is living with brother I can see where staff would think he is the one who should have it.

I would not personally provide care in my home for a parent that I did not have POA for. The last thing I will deal with is a sibling that is not doing the boots on the ground, hands on caregiving telling me what needs to be done or battling me over every penny spent to care for my parent. Your mom chose him for a reason.

Are you planning on taking mom in when she leaves rehab? Because if not, you should not try to take this over.

Seniors end up in the hospital, it doesn't matter where they live. Do you feel like your brother did something to land her in the hospital? If yes, then you need to file a complaint with APS.

Otherwise, be thankful that he is willing to help and show him the appreciation that he deserves for stepping up and giving her a place to live and taking care of her. It is not easy to have a parent live with you.

What date is the POA signed?
Was your mother on that date of sound mind on that date?
Then he is currently her POA and it doesn't matter whatsoever that you did or did not know.
Is your mother is STILL of sound mind, and wishes remove your brother as her POA and to assign YOU as her POA? She is free to do so.
If she has dementia she cannot assign a new POA.
If she was of sound mind when she signed, but now has some dementia, she may not remember having signed this form. But the form would remain legally intact.
If you have reason to believe your brother is acting fraudulently or abusively you can open a case with APS or you can see a lawyer about applying for guardianship.
Guardianship is a costly and frought undertaking. Expect to throw about 10,000 away in any contested application before the court and know that Judges do not look kindly upon families at war over an elder who is still living, and may in fact take guardianhip from the family and assign it to a Fiduciary who will act for Mother's best interests accountable ONLY to the State and to the court. At that point you will have ZERO to say about Mom, including placement and investment, expenditures of her money.
Tread carefully.
Just today Geaton posted in discussions Family Arbitration information. This sounds like a good use of it. If you can't find the post let me know by private message and I will send you the info.

It’s not the law. In fact there’s no law that says any of the siblings has to be involved or chosen as POA. Your mother, as long as she is in her right mind, has the sole discretion to choose her POA and does not have to discuss it with anyone. As far as retrieving her belongings and car, if she assigns you POA then you can ask the police to come for a civil standby but they can’t make him give you anything. They will just be there to keep the peace.

She can decide to change her POA if she is in her "right mind",, but she is correct there could be fallout from him, Are one of you able to take her in if he blows his fuse? If she is in rehab you can see if they have a notory or you can call a lawyer to go to her. The local police can help you get her belongings once the new POA is made ( at least in many areas they will assist, even if just to be available and seen)