Stop suggesting those gross things step-brother. Ewwww... I would never.

Seriously though, can you blame porn? Incest is one of those things that, so easy to include in videos. You just say "step-[relationship]" at any point in the video and BOOM you've now attracted another entire audience.

It's kind of like family shows lately that insert a token gay scene or character. It takes all but 0.5 seconds for them to deliver the dialogue that says "ah-hah! they ARE gay", it doesn't affect the plot in any meaningful way and now disney, pixar exec's get to be like "yeah we've always been down with the rainbow, we are inclusive."