My sister is forbiding me from visiting our mother in the assisted living facility. How do I deal with her controlling behavior?

I looked after mom and dad in NJ for many years. After dad passed, I cared for mom (dad had done everything for her). Her house was destroyed during Sandy and my sister moved her to an assisted living facility in her state (IL). I want to visit my mother and my sister refuses to let me. She is going on an extended vacation and I wanted to spend time with mom while she was away and she is forbiding me from doing so. How can I see mom and not face the wrath of my very controlling and overbearing sister.