WALKED INTO INTIMATE MOMENT

My younger 18 yr old sis has her "almost bf" over, its her 2nd time being face to face with him. My mom wasnt comfortable w them sharing a room so she said they can sleep in the living room. I warned her I get up to pee lots cuz I didn't want to walk into anything. Well I saw her, performing "fellatio". I quickly ran to the bathroom, making noise as I got in, so they knew I was up. I know she's old enough to make those decisions, but I also know she's not on B.C. how can I broach the subject without humiliating her? Also for being dumb enough to do something like that in the open where anyone could see. So awkward, but I want her to be smart about that stuffs.