Had sex with my sister's friend...

So... My sister brung her friend over Saturday night. He was going thru a breakup and well, I'm single af.   Anyway, they brung beers, we got drunk and well... We had sex. The thing is, he was gonna be staying over a couple of days.. Ofc, my sister had no clue of what happened, or at least we thought so. Apparently I left scratches on his back and somehow my sister realized. Later the guy told me my sister asked him straight up and he obviously denied anything happened.