@Lovesssweets if two people, whos parents married, never met each other until they were adults, are they siblings? Te title is there, step-siblings, but are they even? Have they ever interacted as siblings, or thought of each other as one? Not really.
If you arent blood related, dont grow together (childhood), dont even met, not even think of each other as siblings..... they are not siblings.
The only thing they have in common is that their parents lived together, not them.
Its even depicted in the story, the fl only calls him his brother as an excuse to what shes doing, she does it to not get killed. The ml only calls her because of the title, he even says he never thought of her as one. None of them consider the other as a sibling, and distrust each other while maintaining appareances..... then the relationship develops.
I understand what you mean tho, i was very disappointed when i learned about it, but i guess that it is what sells.