How can we help our sister, who is becoming resentful and burnt out due to her role as caregiver for our elderly parents?

I was planning on moving my parents nearer to me so I could be the overseer of their care, but my dad became very ill, and we had to put their move on hold.
One of my sisters lives 5 minutes from my ill and aging parents. She has taken on caretaker responsibilities because of her proximity. My other sister and myself live 4 hours away and visit often to help. But when we leave, it's my sister there that takes over. She is overwhelmed and resentful because we can leave and she can't.
How do we deal with her anger and how can we relieve her of some of the responsibility?