@Karmichel I don't think they are. Far as I can tell, his dad and her . . . mom, was it? both left their respective spouses to run away together, after he was already born. Then his dad came back after a while and has been acting like nothing ever happened, but OL's relative never came back at all. So he has a home put back together at some time before he was really paying attention, while OL's remained broken. So his mom is not someone who ran away--she is as much, or more, a victim than anyone.
So OL has reason to be righteously pissed off at his dad, but as she says, it's not his fault.
We don't know of the runaways having any kids while his dad was gone, but we also don't know there are none--somebody could turn up at some point.
@Karmichel They aren't step-siblings in anyway. The father and mother ran away together (its not clear if the mom left the office lady behind or took her during the "elopement"). The story is implying they share a connection, as children whose parents had an affair with each other, but one was left behind while the other got their father back.
I am really liking Kaede-chan as a character. Cute, smart, cheeky. Her confident and energetic personality make her a good foil for Naotatsu's grounded, reflective character. They're allies right now - wonder if anything more might come?