Considering Yuki never seemed to marry Yumi's dad, it's likely that one of Kaori's parents were her sibling. Kaori wouldn't actually be her niece otherwise, and Yuki tends to say that they are related instead of related by marriage, etc.
I don't recall if It's stated which of Kaori's parents were related to Yuki, but they definitely seem to have died while Kaori survived...sort of:
Kaori definitely didn't seem to come out of it unscathed, and I'm not entirely convinced she's not some kind of zombie, that's new to this timeline. Maya Prime never heard of her.
It'd be a little wild if Nozomu was Kaori's dad, and the one driving the yellow car, or something. Kaori was around 15 when the accident with Sekai happened, and there seems to be some connection between Sekai, Kaori, and Ami, etc.
Kaori has been fond of Ami since the start:
Nevertheless, Nozomu remains a mystery, as does Kaori.
This reminded me that Yuki kind of has 3 daughters. Yumi (bio), Tsuneyo (Tsu-chan) who she feels protective of, and Io, who...well, at least seems to see Yuki as important enough, to her, to randomly bring her groceries.
I wonder how Yuki would react to the recent Io stuff? Nodoka is just begging to be Yuki's punching bag, albeit she might be into it.
The pedo stuff seemed more like a kink than a fetish. Sekai was fucking Nozomu after all. Plus also seemed into girls as well as boys.
She seemed to have done it more because it was wrong, instead of her...type.
Sekai's relationship with Sensei seemed to start off as simply an indulgence, but then became an addiction, and eventually they fell in "love":
Presumably, it's why Sensei was fucking Maya even after she hit puberty. Albeit, he seems to be even more into it than Sekai, considering his temptations with Uta's pic, and Tsukasa recently. Sensei seems like more of a pedo than Sekai actually was.