Yeah, definitely. So my current theory is that Ayua is Miru and William's daughter for sure. Nojiko is most likely Miru's sister meaning that she's Ayua's aunt. If Nika does turn out to be William's child or a Zane is some fashion, I'll be really looking forward to how Ayua and Nika react to the news. I get a feeling she'd be over the moon and probably end up being quite protective.
Nojiko being Miru's sister could be a thing, I agree, but if that's the case, to me there's no ways she doesn't know who Ayua is, and what link she has with Nika/Nami, although there's a lot of secrets kept under still. That's why I see that scene at the book club being so important, there's a power game going on there for sure, but it especially focused on Nojiko in that scene, how she kept things secret to Nika/Nami, and how wary she was seeing Katie there, and surprisingly, how she didn't mind having Nami there, but she especially didn't want Nika at the club.
Also, again considering how promiscuous William is/was, and how things can probably turn around in WiAB, given how Ayua is a Zane in name, but not Nika nor Nami, wonder how Miru reacted to all of this, how she sees them, and how she raised Ayua. She could be over the moon like you said, or she could become defensive. Who knows, maybe William managed to fuck everything up... in every ways possible, of course.