I am also a RN, and have found that the simplest way to handle a case like this is like so:

1. You need to tell her directly, with a witness that what she is saying about you is unacceptable and makes you uncomfortable. I would not get in to an argument either way.
2. If she does not stop, go to the supervisor.

You likely will not change her mind on the matter, as she will not change yours. But, she is a professional who needs to act as such. You need to remain above board, and not stoop to her level.

Every time RNs get in these catty pissing matches, it makes the whole group look bad.