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 in a class (or at least Chika)? 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.
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.
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.