I agree entirely with
@Jaded'n'faded - I don't understand what the benefit of this to you and C.
You only have next of kin status for hospital visits due to POA, you won't be able to have any input into any choice of C's carehome, or any aspect of her care.
I'm not sure it'll keep them quiet but more has the potential to completely exclude you from C's life.
I suspect that C no longer has capacity, and therefore can't remove you as an attorney or appoint anyone in your place even if she wanted to. The H & W POA only kicks in when capacity is lost.
We all know reading your thoughts on here how much you love and care for C, and you are the person that knows her and knows what she needs and the one that will be willing to fight to ensure she has what she needs.
I suspect that her dau has put pressure on the brother, and I'm sure they both feel excluded, but health professionals will only deal with the POA and if on this hospital visit they've followed this correctly then this might be why this has happened. The SW confirmed that as you have POA it is your decision, it is highly likely that they've been in touch and told they don't have input.
I'm not sure it'll give you an easy life in the way you want. C appointed you to act, if you were married there would be no question here of your rights and bar a piece of paper you have lived as a couple for a long time.
Please look after yourself by treating this as the manipulation it is and refuse to countenance it.
Sending ((((((((((((hugs)))))))))))))))