You need to take a cold shower.

That's fucked up.

If you had a step sister, she probably wouldn't want to have sex with you, assuming she wasn't as screwed up as you.

If it's a step sister/brother then it isn't 'fucked up', because they aren't really related. No need to be bitches.

This is fucked up

One of my old school friends a few years back had sex with a boy... then her mum and his dad got together a month later, they turned into step bro and sis and now live together.. awkwarrrddd

if not blood related very hot then maybe

Something like this is a common fantasy. I'm an only child and my parents are still happily married (first marriage), so it is unlikely that I will ever be in this situation, but I still play out fantasies about it in my head.

There are a variety of reason why engaging with a non-blood-related family member would be a poor choice, but as it sounds like you're not seriously considering this I don't feel the need to outline them.