One of my friends is talking behind this one girls back...its really annoying.?

[? j???Y ?]

So one of my friends.."Cathy" always talks about this other girl "Sara." She'll make fun of her and insult her and spread rumors and say how much she hates Sara. But then, she'll go and hang out with her. Its really getting annoying, and it makes me wonder if Cathy does that about me...like what if she tells people rumors behind my back? i doubt it, but idk....Anyway, how do I get her to stop. I don't want to get into a big fight or anything, but god...you either like some one or you don't. (we're in 10th grade if it makes a difference...)