Did I help her cheat or am I good?

I hooked up (went to third base - yes I am also a girl) with another girl a few months ago - I was single. I found out the next day she had a boyfriend of three years, she then proceeded to fuck my little brother. Did I help her cheat / am I also a cheater? I've been wracking my brain about it a lot cause I've seen a lot of girls on here blame the other person but I didn't know. I guess I could've researched her more? But I didn't think I was going to see her again.