My sister is manipulating my elderly parents for money.

I have been taking care of my parents for at least 10 years. The past 2 years my mother has been in and out of the hospital and my father has short term memory loss problems and is confined to a wheel chair. I do have Power of Attorney on both of their finances and overall health.

I have a sister who manipulates my parents into loaning her money. She constantly threatens to not visit them and not let her see her grandchildren. Lately my sister was mad at them because they comment about a picture her daughter placed on facebook My parents never commented about the picture online, just face to face asking about one of her daughter's friends. It wasn't improper in any way. Neither one of them can visit my sister's home because they are unable to leave the home. My sister disowns her parents and myself when she doesn't get her own way. Many time my sister has verbally abused my mother over the phone.A few months later she will start talking to my mother again and start asking for money which she almost never pays back. A few years ago, my sister owed money on bills she didn't pay and listed my parents phone number as a contact number. I had a heck of a time stopping the bill collectors from calling our home. My sister makes around $30 per hour and borrows from my parents which their fixed income together is $1700 per month. She's working on her 2nd divorce and has a boyfriend on the side.She's 42 years old.

I would appreciate any advice on this matter. What coarse of action should I take? I have ignored this several times and it always seems to repeat itself.