Didn't something come up about your name on with your mom a couple of years ago? I think you posted about it..is this the same account?
When my mom made out her POA for me (and also my little old lady we take care of), my hubby was named as second, in case something happened to me. Perhaps that's what happened here...your brother had it, and then his wife took over if he couldn't. And if it's a full POA, your sil can have a ball, if she's not honest. If she was the exector, she would have to let you guys know what she's doing with the money, and have an accounting, but as a POA, she doesn't (unless your mom is about to apply to medicaid, then they get involved with the percentages spent..but I believe you said before that isn't the case). What you might want to do, while waiting for a lawyer, is get your mom to assign you or your brother as a POA, and revoke the sil.
Then, kick your brother in the butt..you tried to tell him to take the accounts, but noooo, he wouldn't listen.