What am I responsible to give my brother who is questioning the money I spend for mom?

Your obligation is to your mother. She made you POA. You should not share her private information with anyone except those who also have a fiduciary obligation to her such as an attorney or investment advisor, etc.

Well, they actually are getting something extra -- the responsibility. ;-)

Do keep a careful accounting however. You can't use the money on yourself so keep a good ledger (just in case he contests something along the way.)
Beyond that I believe it is inappropriate to discuss Mom's finances as you now have a fiduciary responsibility to her.
I neither  expected (nor had)  any issues when I was POA for my Mom. Still I was buying items and reimbursing myself by check. I kept detailed lists and quarterly (unasked) I sent an itemized list of all purchases for her along with the check number I wrote to myself in that amount. I was not the executor of the estate but I handed over all of the receipts to the executor.
Non caregivers are always afraid the caregiver is getting something extra. (sigh) family life!!

Legally you don't have to give him anything. It might smooth the waters if you would share with him. It wouldn't bother me to share information with my brothers if they were to ask. They don't, since the money probably doesn't matter to them and they can see we live simply. They are heirs to any future estate, though, so I wouldn't mind if they knew everything that I do. I trust them not to misuse any information. They're very honest people.

You are responsible to give your brother nothing. You as POA have a financial obligation to protect your mothers finances to include not divulging account numbers to a third party without a court order or your mother's say so.

You do not have to give him anything. As POA, you need to spend her money on her care in her personal best interest, but you do not have to prove that to him. He has no say in whether or not you are spending her money unwisely. He would have to take you to court an contest the POA. Tell him you're happy to show him everything you have bought for mom the next time he visits.

I believe I need to show her checking acct and tax returns right? he is so afraid I am spending her money!!

yes but as POA I need to prove I am not spending her money unwisely he is shrewd!!

NOthing - it is your mom's money, spent for her care. Tell him to pound sand