Denial is a stage that can last awhile - especially in a death that was unexpected. I am sure that the family of those killed at the World Trade Towers reacted the same way.
If she is asking you the same question over and over again it is because she needs to hear the same answer over and over again until she can safely emotionally allow it to sink all the way in.
I would reply as compassionately each time she asks as if it was the first time.
"I am so sorry Ellie, your sister was in a really bad accident and she didn't survive. They did every thing they could and now all we can do is remember all the wonderful things about her."
If she is a person of faith you can remind her that the important parts of her sister (her personality, her soul, who she was inside) is still here.