[relentlessflame]

Originally Posted by iCK

Spoiler for Countercounterargument:

It also explains in kirino's true route that they aren't blood related siblings.

Spoiler for Correction to game comment:

No, not in the True Route, but in the "If" Route, which is an alternate take on the story. The very fact that it's in an alternate universe where they're not blood related suggests that they really are blood-related in the story proper.