User blog comment:Xilinoc/Ch. 651 - The Theatre Suicide SCENE 5/@comment-29449108-20151113223353/@comment-26091912-20151114004532

There's a good chance that Nanao's father isn't Shunsui's brother.

In that little flashback, Shunsui looked pretty young; a bit older than he was when Yamamoto caught him in his office. Still looked much younger than when he was in the Shin'o Academy, so we'll say about 900 or so years ago.

If the men that marry into the Ise family die shortly thereafter, we'll say that a short time by Shinigami standards is about 50 years; 100 years max. Nanao herself, I would say is no older than Rukia, so 150 years old.

So if Shunsui's brother really is Nanao's father, then that would mean he and her mother would have to have been together for at least 700 years before Nanao was born. Which hardly qualifies as a 'short time' even by Soul standards. Unless we assume that Nanao is 700 years old, which is highly improbable.