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)

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

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)