I was totally going to make a topic about this. I think its just a progression of the whole milf thing. A few years ago milf porn was everywhere. Looking at it from a psychological stand point and subscribing to the notion that porn is addictive, the movement to incest/step-family porn is just another step to scratch that itch.
I had a hot stepmom come into the picture when I was around 12-13. I won't lie, there were times where I tried to sneak a peak when I knew she was changing or in the shower. I didn't have a stepsister, but i'm sure the same feelings would be there too if she was the same age or older.
I could go on and on... because deep down i'm completely fucked up lol but I think there are a few reasons why its becoming more prevalent, whether you are scratching that itch of getting more taboo, something from your past, or just because its the only thing there.