Is my rat dying or just sick?

Typically rats only live until 3-4 years of age. It sounds like she is dying, you should take her out so her sister isn't bothering her and have her last moments with you. I had a rat that waited until I got home from work to pass in my lap. It was so heart breaking! I would keep her on the heating pad and keep her comfortable. If you can't bear it, you should have her put to sleep.