My parents are failing and sister has taken control of finances and possibly gotten POA. What can I do?

I have taken care of my parents for the last 3 years, every day. They were just placed in assisted living and their mental status is diminishing. I just suffered from a major heart attack and during my time in the hospital my older sister has gotten on their bank accounts, taken their identification, will not answer direct questions about my parents finances. Their house was just sold. She has lied to me on various occasions and will not agree to sit down and talk to me. I believe she may have coerced my parents into signing a power of attorney. Unfortunately at this point I don't think they would remember. I have always been very close to my parents and my sister has not. I was their primary caregiver for years. I do not know which way to turn. Any advice is greatly appreciated.