What's the softest way to tell a guy he's a cuckold, because my brother's already in me

Sister got cocky, let her live for a blowjob, and she's already riding his dick (part 1)

Fucking my brother in the dressing room, because we're out of room for sex at home (part 4)

I went into the shower and my sister was there - I didn't get confused and went into her (part 2)

I didn't know my sister liked it rough - from this day on I fuck her only this way (part 3)

I won't let my sister get naughty, I'll fuck her so she won't sleep with anyone else (part 5)