IIRC it’s Tsubaki’s mother that is the possible daughter of Renji. I think it was implied that he accepted her as his child without question but that it wasn’t necessarily true. Whatever the case, she was raised in the family and is Yoshino’s aunt, therefore Tsubaki is her cousin. For a few years Yoshino was sent to live with them to get a female influence.
Tsubaki just started college, and takes guys lightly (dates/sleeps around a lot) but has a mega crush on Renji. Yoshino resembles him a bit, so she dotes on her as well. There is some connection to medical organizations through her dad, so Tsubaki helped out during the whole “selling a kidney” episode.
Yeah the language is actually pretty confusing— it’s in Volume 3 Chapter 12 (p90 if you have the licensed version) the mistress is Yoshino’s ‘half-grandmother’ and her daughter is Kyosuke’s half-sister so she’s Yoshino’s cousin (and by that logic, Shouma is Yoshino’s uncle so riiiiiip to that ship). But no, according to Yoshino he didn’t verify but raised the aunt (who was 10 at the time she was presented to him) as a daughter and Yoshino considers her an aunt. A lot of families call close family friends ‘aunt’ or ‘cousin’ regardless of ages, so it’s not surprising Yoshino would be resigned to all this, aside from her being aware that Renji is ‘prolific’ or whatever. In a Pixiv sketch Konishi said Yoshino went to live with the Akashigata family in junior high to get a little more socialized and apparently Hotei was pretty devastated.
Tsubaki’s mother is in a relationship with the CMO of the Kyoto Doseikai hospital and board member of the Doseikai group. Tsubaki refers to the unnamed man as ‘Father’ to Kirishima but also says her mother isn’t married to him, they’ve just been in a relationship for 15 years (most of her life so it would make sense to refer to him that way).
Kinda hoping Tsubaki makes it back into the story at some point because there’s some fun stuff going on there with the parentage thing, the daddy issues, the involvement of the Doseikai group in organ selling, and the fact that Tsubaki TOOK ALL THAT BLOOD AND DIDN’T TELL YOSHINO and stuff. Also the ‘I’ll get her plastic surgery so you never find her again’ thing if Kirishima screws up. Enough there for at least a side story like Futari wa Teihein, imo.