Yup...I can attest to this...SIL scammed MIL for years...
In our family too. I finally moved her money when hubby got POA to another bank where SIL couldn’t touch it. When it came time to share the inheritance I deducted what I had proof she had taken. She didn’t say a word because I had politely warned her that she could be prosecuted.
That was only the money she took w/o permission, it didn’t account the 10s of thousands she begged off her mother for all of her adult life.