Need to find out what I can do as my sister's husband is very involved with my mother's finances and we want him to step away.

If Mom is not satisfied with the way the financial POA is being handled now, she can assign someone else. If she is happy, I don't know there is a lot you can do.

It is the health care POA who has the authority to decide where Mom lives, BTW.

Who is primary on the HCPOA? In most states, that is the person driving the bus.

If mom is competent, she can make her own decisions. Including excluding BIL from having an influence. Has anyone consulted an eldercare attorney?

Well, they like to spend $$s that are not necessary, they don't consult me which I am 2nd person on the HCPOA, they've (and he is not listed on the HCPOA) made suggestions that they want to save money rather than putting my mother in a facility where she would be well taken care and implied that their inheritance would be less.  It's not his mother, it's not his money and there are 3 of us sisters I feel should be making the decision rather than my sister' husband.  My sister even said that he can talk my mom into anything and laughed rather than her.  We are even paying my other sister to care for my mother to "save money" and she just texted me saying they want to raise what she is currently getting.  She has no medical background, keeps a dirty house....  What is reasonable?  There is just so much that has red flags to me.

Just thought of what my mother had said that she didn't want him involved but she does have some memory loss so one day she might say that and another day, maybe not.

If mother split the POA, perhaps she was expecting BIL to be in there with your sister because he has particular experience? What is the problem? Do you think the advice is wrong financially? Do you think he is being self-interested in the advice, or taking advantage of it in some way? Or is he just a know-it-all pain in the neck? You need to give us more information about the real problem.

What in particular is upsetting you?  Your financial POA sister may be perfectly happy to work with her husband.