Recently I (m17) had sex with my brother's ex (m20)(f21), they dated for a summer 5 years ago when they were 15/16 so it wasn't that big of a deal.

Recently we were together at a party and we were high and drunk and happened to end up having sex, I wasn't even thinking about her as my brother's ex bc it's been a long time, but I feel so bad and I don't know if I should tell my brother, I don't intend to have any more relation with her, so I'm not sure if there's any point in telling him and creating an unnecessary argument.

What should I do? Am I the asshole?