My mother is very ill and unable to sign a POA. How can I protect her interest?

An individual (not related to my mother) has complete access to my mother's finances (e.g., Checking Account, etc.). Got my mother to sign off on some docs at the bank, under false pretenses. This person is a sister-in-law to my brother. My mother thought she was doing the right thing by having my brother oversee her finances, but his sister-in-law has COMPLETELY TAKEN OVER...my Dad & I don't know what's happening with my mother's finances. Most recently, I found out, my mother wasn't properly being cared for. I NEED HELP IMMEDIATELY!