Ignore it!

You are in the middle of sexual release and too horny to think much, your only thought is, if you look up and are caught, it will kill the mood.

You keep at it, fingering yourself to orgasm after orgasm, between songs you think you hear a fapping noise but at this point you really don't care enough to stop.

eventually you decide to stop, after 8 or so orgasms you rest. you hear a moan and feel a hot liquid hitting your uncovered vagina, repeatedly.

At this point you assume it's one of your brother's friends and decide you would rather not know who so you keep your eyes closed until you hear him leave.