Step stuff isn't an incest like step brother but step father is an incest... ( we're talking about yaoi?) if you're talking morally normally people wont fall in love with father,mother,brother,uncles and aunts and in some cultures cousins... and morally step stuff. But personally I like yaoi with step brothers and I can't see it as incest.
The only way it can be called incest is if one of your parents' remarries and has a child together. Say your mom/dad has you and 1 sister/brother and the other person has a son/daughter from previous marriage, it wouldn't be called incest. If you've watch soaps, that would be exaggerating, but somewhat true when the story lines' created in that way. However, ex. if your mom has a kid with your step-dad, that child would be your half-brother/sister and it would still be consider incest that child would have half your blood relation and that would be risky.
I think step-relatives depends. I mean, if they're 18 and have been step siblings since they were 5, that's a hell of a lot difference if they're 18 and have been siblings since they were 15. Cousins I'm okay with since, while it's frowned upon in my country and even illegal in many places, I understand that it's considered acceptable in some places. Step-parents I have a problem with since they married their spouse and promised to be a father/mother for their step-child. To me, a romantic and/or sexual relationship between them, to me, seems like a betrayal to both the spouse and the child.
Surprisingly it's ok if you've got an illegal child who doesn't know about you, later on you 2 can marry. It may sound stupid. but take a moment and think, he's got a child never seen and finally they meet they don't know they're daughter and father and got married? Laws wont forbid this relationship to continue.. So I think in cases like these where people don't know they're blood related and fall in love is just to let them be.
And as stated above, being step brothers /sisters when you're 5 not like when you're 18...Personally someone I'm growing up with will be like a real brother so no as well.
Step parents/ sons and daughters in law are well, idk personally.
In the case I mentioned, they did know they were blood-related beforehand, which is the problem. But different from @LessThanThree's case, only both sides of the family knew about their situation. I found an article with more detailed info
http://www.dailymail.co.uk/news/article-2915094/I-lost-virginity-d-never-felt-comfortable-man-Daughter-plans-marrying-father-talks-taboo-relationship.html
Also, as someone who's basically a daddy's girl despite being an adult, I find this especially disconcerting. He shouldn't be having kids. We've already seen what happens when he does that. Even if the baby doesn't have a disorder, it's just wrong. Oedipus thought it was wrong. When a guy in Ancient Greek mythology has a better sense of reasoning than you, there is a problem. XD
Am I the only one who think that incest is happened when 2 or more people doing it if they're blood related only? And think that step-something doesn't count as incest? :