Rin skipped a year when it comes to the rest of the girls:
Well, that returns some credence to the theory. I'm supposed to believe it's a coincidence Rin just happens to be that much unusually younger than the others? I'd say it's more likely she, at the minimum, shares a mother with someone (Ami). Otherwise, that's quite the unused plot point.
Rika - "Nah. Douchebag ran off before she was even born. Wouldn't be surprised if he already had a family or something like that. Even the case-worker didn't know what his deal was."
(Chapter 3's Rika 'Impregnation Spree' Event)
The impression I got from Rika, combined with the other statements, is that she also didn't really know and was making assumptions. All she actually seems to know is he wasn't around when Rin was born. Any reasonable adult (inc. Sara) would assume it's because he abandoned her. Selebus leans quite extra heavy into how much they're making assumptions, and even those who should know appear to not
know.
Reading back through, I see discussion about Nozomu being Akira's brother? But I can't find messages discussing evidence on that. (I'd like to see more).
Regardless, at the very least, I think Rin is Sekai's with a considerable lack of info.
but the existance of another child is dubious at best simply because it would conflict with Sekais charater.
Well, thing is, that whole period of their lives (Akira's teenager years until becoming a tutor) is, to my knowledge, a blank in the story right now that Akira refuses to even think about. Sekai, for instance, could have been forced by circumstance. I agree it wouldn't make sense for her to willingly give up Rin.
EDIT: That said, it would be weird that she'd have to give up Rin but not Ami. I have no idea how that plot would come to be.