How old is the cat? How old is your sister? Was your sister doing anything annoying to the cat that caused it to strike out and bite? You've left out a lot of details in this story.
If your sister was bothering the cat, then the cat has the right to defend itself. If nothing was going on and the cat decided to bite her, then I can see how your parents would be concerned. However, there aren't enough details to tell you for sure what to do.