You may find the answer to your question in this complicated situation by looking at the actual power of attorney document that appointed you the agent"”and by paying close attention
to the timing of everything involved.
Most powers of attorney specify when and how they will take effect. Often, they state that the document goes into effect when one"”or sometimes two"”licensed doctors verify in writing that the person for whom they were made is no longer mentally competent. If that happened, then there is a good argument, as you note, that the first power of attorney is in effect and that your parents actually lacked the capacity to revoke it and make a new one.
But if the document didn't specify when and how it was to take effect, then they are free to make a new power of attorney as long as they had sufficient legal capacity. This may be a lower standard than you would guess. It requires only that a person understands the subject area covered by the power of attorney, understands the implications and importance of the matters involved, and can make and communicate reasoned choices.
If the attorney you consulted was involved in helping your parents write up the new power of attorney appointing your brother, then presumably the attorney thought they had the required mental capacity. If you want to challenge the document, then you will have to go to court and present evidence that they lacked that capacity.
But all this legal battling takes time and effort. And while it sounds as if the whole matter of the power of attorney has led to some hard words and hurt feelings in your family, you might simply take a hard look at what needs to be done to keep your parents living as safely and soundly as possible. If that's happening, then the whole issue of the power of who has the power of attorney may not realistically matter much.
Look at whether there are any needs that are going unmet because of this confusion. If not, put the matter behind you. If there are real issues"”the bills are not being paid on time, for example"”then raise that issue with your brother and parents. Working together, you might find a solution"”such as automatic payments from a bank account"”that gets the job done.