Of course they were never actually siblings. Why did the author want to write this even? The only difference of this with other stuff is that they thought they were twins not just mere siblings. And if we're going to talk about it, Yosuga no Sora actually has the real thing fornicating.
Why fake twins?
Why must they me twins is passing them of as siblings not enough?
And having the parents be divorced and them living apart when younger but now living together again,
or something like that would make it be more sensible.
or just go full wack and have them be actually related.
@RanaRana If they're about the same age they would need to make one of them believe he was older or younger, at least 10 months apart from the other, so they probably went to twins, which in case of different gender, its always fraternal twins, just regular siblings who happened to be born from the same pregnancy