Can my husband protect his Mom even if his brother has POA?

My mother-in-law is living with my husband's brother who was given the POA rights to her after her husband died. We just found out two years ago and he died in 1992. To make a long story short, she came to stay with us in April 2011 while they went on vacation. She did not want to go back with them because she said they were mean to her. Well after a little over a month, she got mad at my husband (her son) because he was trying to help her reduce her sugar intake because she is a severe diabetic. Anyway, during that time we found out that our sister-in-law has been spending her money right and left. Her son by another man, stole checks from Mom and they ended up freezing her account. From January 2011 to April 2011, she had over $34,000 deposited to her account. By the time we got her, she had $24.86 and her account was frozen. To make a long story short, when I took her to the bank the manager gave us a four month account history. My sister-in-law had personal bills such as credit cards, cell phone bills, etc. coming out of Mom's account. When I mentioned this, I was told to don't bring it up or my husband's brother was going to commit suicide. Then she threatened to attack me the last time we were over at their house to visit Mom because I exposed her. She is now bedfast and I know she isn't getting the care she needs. However, everything that has happened has been pinned on me. I sometimes wish I hadnt seen anything or said anything because nothing really changed except we can't see her now and she has been brain washed by these idiots that I am a bad person when I was just trying to help. She is getting worse by the day is what I heard. I don't believe they have her best interest at heart. Is there anything my husband can do to protect his Mom even though his brother is the POA?