I see myself looking in on the action. Hiding to watch it go down. Same with Paternal incest, but with siblings, I don't really have to imagine.
Oh yeah, I forgot to mention: I don't have to imagine the mother. They tend to be the ones described in the most detail (And never look like my mom, thankfully. No offense, but "Ew." I used to tell "My mama so fat..." jokes in Jr. High.)