Bros, I know this is going to sound too crazy, but as god as my witness (thanks big guy) it is the truth.
I have an older brother. My Mom married this dude with two daughters roughly the same age as us. The girls lived with their mom, but came to visit on the weekends. When I was 16 and my older bro 19 we were CONSTANTLY tapping our step counterparts. The crazy part was, eventually, we SWITCHED. I was banging the older one and started banging the younger one. We are all married now (seperately!!) and share laughs about it in secret!! Easily the hottest thing...minus being where my brother was.