Friends brother

So I have a friend who I've know for a SUPER long time and I've known her brother for the same amount of time. So 2 years ago her brother and I started messing around and it was pretty casual, like it wasn't really a big thing. So recently my friend found out about it, and won't talk to me. She's super mad and I mean I guess I understand, but no one really thinks it's a big deal. Her brother doesn't care who knows and doesn't think it's a big deal, neither does his now gf who found out too, neither do I, but my friend does. His gf suggested I talk to her with the 2 of them and try to work things out, and I did, but she still won't talk to me. Everyone is telling me that she told them she wants me to apologize, but for what? For messing around with him 2 years ago? For not telling her? I only didn't tell her because she's REALLY irrational, but i don't really know what to do. Wait for her to get over it, apologize? I'm not sure how to handle this situation with her. Any advise???