Board Thread:Improvements and Issues/@comment-5355747-20130313062554/@comment-6935068-20130710095231

@N-e-ls : First cousin? What did you mean?

Well, I can answer most of your questions. First, let's build the timeline, okay?

1. Year unknown - Isshin Shiba is the captain of the 10th Division, possibly he was already a captain even before Toshiro meets Rangiku.

2. Year unknown - Several years after Momo Hinamori studied at the Shinigami Academy,Rangiku Matsumoto met Toshiro for the first time. Invited him to become a Shinigami.

3. Rukia "killed" Kaien sometime after she joined the 13th Division.

4. 20 years before Bleach's prologue - The "Memories in the Rain OP 1-10" starts. At that time, Toshiro had become the 3rd Seat of 10th Division. Known Head of Shiba Clan : Isshin Shiba

5. After his Shinigami powers sealed, Isshin Shiba was relinquished of the 10th Division Captain position. His title as the Head of Shiba clan was also stripped.

The title as the head of a clan is usually held by the eldest person in the clan. Let's just say that Kaien is the eldest. So, If Isshin is missing, and he doesn't have any brother or sister, nor even a child, then the position will be held by his cousin.

Kukaku will eventually gain the title as the Head of Shiba Clan (20-1 years prior the Bleach's prologue) after Isshin went missing, as you said. Though it's not stated directly in the manga.

So if Ichigo's not their cousin, who's Ichigo? Isshin doesn't have any child older than Ichigo, right? Kukaku, Kaien, and Ganju is his children, then Kukaku will call Isshin her father in Chapter 518.