Is it normal that i slept with my best friend's brother?

A year ago, I slept with a close girlfriend's (semi-frenemy) twin brother, knowing that she wasn't okay with it. He and I also talked for a long time behind her back. We hooked up at a party that he threw while she was out of town (she threatened to leave my friend group just because we attended said party-- I can't imagine what she'd do/think if she knew that I had sex with her brother that night). A year and 4 months later, she still doesn't know. There are upwards of 50 of our mutual friends who know, and they all took our side and chose not to tell her. I think that's the part that would hurt the most if she found out.