We first had PoAs when we were in our 40s. My sister had just died in a road accident and another road accident involving a family whose children were at school with mine, made us realise that we needed to be prepared for all eventualities.
We first made wills after our children were born in order to ensure who would bring them up if anything happened to us and ensure the financial arrangements.
As others have said, making the PoA gives your brother no powers until it is activated. Your mother is in fact being very sensible in making sure that if she suddenly becomes unable to look after herself, age is no protection against strokes heart attacks or accident, someone has powers to represent her interests.
I think the reason she is doing this now is to avoid the conflict she fears would happen round her bedside between you and your brother if she became incapacitated not having put one in place, or since, you state the she is cause of the estrangement, because she does not want you to have any say in her care if she becomes incapacitated.
But that aside, basically there is nothing remarkable about a woman of her age making a PoA, many people much younger than she is have already signed such documents. It is a sensible thing to do as she reaches old age.
There isn't really for much for you to discuss with your mother over making the POA. You may want to discuss with her why you have not been included, but, personally, I wouldn't