Before you jump to conclusions, consider this: You start dating someone and become close to them then, after meeting, the single parents meet and get married. The person you're in a relationship with is now a stepbrother/sister.
Should your relationship, possibly physical, come to an end?
And if not, should it be any different if you become physical after becoming step-siblings?

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You'd hope the parents discussed this with you beforehand, so everyone knew this was coming. It's pretty messed up of them otherwise. The relationship pre-dates the step-sibling title, which really is just a title, especially if you're adults, so I'd still consider them to be that rather than technically family and nothing changes for me. I have some step-siblings I was friends with long before our parents got married and I still consider them friends before step-siblings.
So does it make a difference as to which came first?
Very much
I would end the relationship. That would be way too weird for me technically being family members even if not by blood.
Same reason I wouldn't date a cousin. That's weird.
What if you had really strong feelings for him? If you had become married, would you consider divorce?
Why are you moving the goalpoats? I can come up with hypotheticals too. We could do this all day.
Not trying to move anything.
This is only clarifying how close you are to this significant other.
Not for Americans.
no, I imsgine this happens more than you think
What would be the problem, you were dating first
Sure. But what if you were step-siblings first?
That would be a bit awkward
I can understand that, but would it be wrong?
Not really, they are not related