I'm not asking to be difficult, but there is more to being involved in her (current) life than spending time and speaking with her daily. Do you know who her doctors are? Do you take her to appointments? Know what meds she takes and on what schedule? If there are any dietary concerns? Whether she needs any physical accommodations?
As I said, I'm not asking to be difficult, but I am in a similar situation, from the other side. And my brother will also tell people that he spends time with them and speaks with them daily. He does the latter. But there is more to our parents' lives/issues than calling twice a day. The day-to-day care involved requires significantly more work than visiting or calling regularly. Believe me. Not something for the faint of heart.
ETA: You apparently have other brothers. Do they share your concerns?
While I absolutely and positively agree with this response, it honestly is of questionable legality if your brother had her sign a POA if she is not actually competent to do so. The question is, at that point, is what will your brother do with that POA? Will he use it to responsibly deal with your mother's affairs, or will he use it to raid her finances...and/or to otherwise do things that are not in her best interest? Did your brother do that because mom's financial/medical affairs are out of order and it was needed to gain some control to get things back in order, or did he do it out of greed?
You seem to have some concern that your brother did it for some nefarious reason...when its equally possible that he did it in order to save a sinking ship...or perhaps some other reason that neither you nor we have an knowledge about.
Talk to your brother about the situation in a calm and logical manner...or if perhaps you were contributing to the sinking ship, even without intending to do so, then perhaps accept the situation. Or, if you are certain that your brother has done so for a nefarious reason, then by all means consult an attorney.
