What should I do about this girl?

I’m pretty certain what I should but I guess I need an outside opinion. 2 weeks ago an ex and I started to become good friends again. We weren’t friends for the past 4 years because we had a bad break up. Anyways, I still kinda still have feelings for her. It is only 2 weeks and I am not ready to tell her how I feel. Well, she told me today that she’s going to try and rekindle an old relationship. I made a sarcastic comment (which I didn’t mean it to be sarcastic originally). Anyways she knows I don’t like her getting back together with him. Mainly for my feelings. Basically I’m stuck on either telling her how I feel, or trying to avoid it.