Help, brother is POA and may be taking funds. Do I have a right to see how he is spending their money?

"A feeling" probably won't get anyone to take you very seriously. Actual evidence, or grounds for suspicion might, though. Brand new estate car? Five star travel to exciting new places? SIL suddenly has something more interesting to talk about than her mortgage worries?

Have a look at your state/county website and see what you can find out about reporting suspicions of financial abuse of vulnerable adults. You don't have a right to see your mother's accounts, but there are people in authority who do provided they have grounds to demand them - it's a question of rootling out the right ones.

Oh, btw - put all thoughts of your ⅓ funds out of your mind. It has no existence until - spit spit spit may she live forever - your mother passes on. I know you only mention it in the hope that as a potential beneficiary you might have more leverage (you don't, not at this point); but if it's seen as your motive for acting it will win you no friends with the powers-that-be.

From what I have read on here, no, you cannot force him to show you anything. Obviously your parents trusted him as they made him POA. If you think he is abusing that power, report it as elder abuse.