That's a big reason I try to show the screenshots. But actually, it could be the other way around.
I'd think if Nodoka had seen Sekai even as a hallucination, she'd probably realize that Ami and her, look too much alike to not be related. She's just now realizing they are related because their poetry was similar. Probably didn't even know Sekai's last name, until now. Maybe not even the name Sekai.
Otoha on the other hand, well, she's apparently trying to pretend she doesn't know Sekai's pseudonym, even from Nodoka. She also didn't seem surprised to find out she was Ami's mother.
For all we know, she and Sekai are connected. Otoha is virtually the only girl that likes the idea of a little brother, as much as Sekai liked her little brother in law. Both of which, even seeing Sensei as that little brother:
Otoha also shares a few traits that Sekai likely did. Both just happening to be inherently popular, manipulative, and creative, plus they also seem to embrace being shitty people, enjoy power dynamics, and the idea of being with younger boys.
Not entirely sure what the point of Otoha is, but it's becoming likely that it's related to Sekai. She has also seen the Upside Down House though:
So, there might be something there.