User blog comment:Xilinoc/Ch. 651 - The Theatre Suicide SCENE 5/@comment-29449108-20151113223353/@comment-5398287-20151114065832

I, uh...

Alright, let me put forward a theory we had in the podcast. Men that marry into the Ise family die shortly afterward, correct? So in order to stave off that, Nanao's mom and Shunsui's brother (who has to be her father, there's literally no point in mentioning him otherwise) simply didn't get married for a while, then got married and had Nanao before the brother died. At this point, Shunsui is a Shinigami with his own Zanpakuto, and thus his own Zanpakuto Spirit - this allows Nanao's mom to give him the sacred sword for safekeeping, and he hides it in his Zanpakuto Spirit. Let's say that, hypothetically, this is 200-300 years ago instead of 700 or whatever: that's why Nanao would still look like a little girl during TBtP (on top of Shinigami aging at different rates, as evidenced by Hitsugaya).