Should I have been informed when my younger brother became power of attorney for our mother?

To quote Joanne:

"Just...I would not do any caregiving unless I had both financial and medical."

If you get inveigled into on-site caregiving, giving up your job, derailing your life in order to be a caregiver to your mother, you should be paid by your parent (and there needs to be a legal caregiving contract in place) and you need to have adequate back up and respite.

Don't get into a situation where you are the full time caregiver and need to beg your sibling for approval for payment, respite or reimbursement for supplies.

I agree with others, no requirement you be informed by either one of them,  BUT I would be very leery of providing help to your mother.  I would need to be convinced that mom did not have money for her own care (or not eligible for Medicaid).

Only if your mother made that a condition of the POA. Otherwise, no, not especially.

Is it a problem?

Your mom was under no obligation to tell you. The decision was hers alone. She only needs the consent of the person she wants to be her POA.

No. It might have been nice to know, but it isn't necessary. Being POA is basically that someone competent chooses whomever they think might be best in the family to be POA or DPOA if the need comes forth, or if the need is now. One would likely choose a person for any number of reasons. They are good at math. They have the most time to devote. They are dedicated to keeping good records. They volunteered to help. The POA acts for the person as directed. Or when the time comes the person cannot act in his or own behalf then as the person dicatated when well, or as they understand the person would want, or as they feel is best interests of the person.
Records should be kept. No one is privvy to any of the information or records other than the person whose finances and health are managed and the POA.
I would honestly say, be relieved you don't have to take on the job. It isn't easy. Be as much support as you can.

Uhm, no, there is no obligation for either of them to inform you. That said of the two of them it would be up to your mother to tell the rest of the family her plans, your brother is obligated to follow her wishes.

It would have been nice if your Mom had told you but there is no law that certain people need to be notified.

Just...I would not do any caregiving unless I had both financial and medical.