If there was a POA done for Aunt Bessie and now she doesn't trust the POA and wants to change it what should she do?

Without seeing the specifics, Aunt Bessie might be wrong. Get Aunt Bessie to show you the bank records before you rush to judgment, find out where the money really went. If you have a solid case of fraud, call the police.

It might be helpful to have a copy of the current POA which names the SIL. I have POA for my mom, and it states that it is in effect until mom names someone else and notifies the current POA of the new appointment. This can't happen for my mom since she is no longer competent, but if Aunt Bessie has not been declared incompetent, and her POA paperwork is similar, it should be easy to make the change.

Some States or counties require either notarizing the forms and/or witnessing. The forums you find better appropriate with in your state or county will generally have the signature lines showing you what is required in your area. You can also check with the local Department of Aging or with the county recorder to find out if your state or county requires that you file the POAs. Most don't but you'll need to do it if it is necessary.

Ok then, are you in MO too and would be her new agent on medical and financial POA's? Those are the least expensive ways to go, you can get forms and you don't need a lawyer. Make sure the financial POA is a "durable" one. She can also choose whether it goes into effect immediately or only after she is declared incompetent. You will need the immediate type if she wants you to help her handle her business now well she is still competent.

Aunt Bessie is quite competent, and lives alone in Missouri. She has no helath issues and is not on any medication at all. she is just old.

You say that you are in Missouri, but you don't say if Aunt Bessie is there also or what her medical condition as. If she is competent, she can either execute new financial and health POA's or go to an elder care attorney and establish a trust, I'm assuming with you as your agent or trustee as the case may be. If she is not competent, you will have to pay attention for conservatorship (some States call it guardianship) and it can be costly if anyone contests your application. That would probably be her SIL who may fight you, so you'd need to go in with proof about her wrong doing before she disappears with it. In other words, get your ducks in a row before you start quacking. I might mention, if Aunt Bessie is incompetent (usually from dementia), make sure before you set any wheels in motion, make sure you see in writing this money her SIL has supposedly been secret in a way, as dementia folks are quite capable of paranoia and confabulation - fear and mental disarray assists them in easily making up fantasical stories.