Kukaku and Ganju Shiba's Uncle is...
Well at least I think so. I've heard someone mention this before, but I have pieced together more facts to give logical explanation. If you don't know where I'm getting this from in a recent chapter (Ch.518, P.15-16), Ganju asks if Kukaku was ok with sending Ichigo to Soul Palace. Kukaku replies by saying that if Ichigo doesn't go, the Soul Society is will be destroyed. She says that they must let Ichigo go even if it makes "Uncle" sad. Now I've collected some evidence as to why I believe Isshin is Kukaku's Uncle.
First of all I would like to quote an exert from my previous thread on Ichigo's Mom and Isshin.
In this quote I explained how and why Isshin gave up his powers. It was to be with a Quincy, Ichigo's Mom, Masaki. I also explain why Urahara asked if Isshin was ok with returning Ichigo's Shinigami powers at the end of The Fullbringer Arc (Ch.448, P.21).
Originally Posted by EmperorIssun
Now Ichigo has gone to the Soul Palace to rehabilitate himself and restore his Bankai so that he can face the Juha and the Quincies again. Ichigo is getting himself more involved with the Soul Society, which goes against what Isshin and Masaki wanted (Which was to be human, and be with each other). With the conclusion that Isshin does never wanted Ichigo to become a Shinigami or be involved with the Soul Society, I believe that Kukaku is referring to Isshin when she says "Even if uncle will be upset". It makes sense that Isshin would be upset that Ichigo has chosen this path.
Another piece of evidence I would like to address is the Nobility of the Shiba Clan
Originally they were included in the 4 Great Noble families but apparently fell out of Nobility after Kaien Shiba was killed. Now this seems unreasonable, the head of the Clan dies and they lose their status and are sent to Rukongai?
But let's consider how Kaien died. He was possessed by a Hollow and forcefully dispatched by Rukia. Not exactly the most honorable death if you asked me. I think it might be considered shameful that he died like this. If we factor in the theory that Isshin is related to Kukaku and was maybe even the head of the Shiba Clan. Wouldn't his actions (defection from the Soul Society, removal of his powers and love for a Quincy) be considered a shameful act of desertion? So since both heads of the Shiba Clan dishonored the name of the Shiba Clan, I believe that this is why the Shiba Clan fell out of its high status.
Finally a minor detail (or crucial hint) that has come to my attention is the similarities between the appearances of the Kurosakis and the Shibas. It might just be me, but I think Isshin looks like Ganju, Karin looks like Kukaku and Ichigo looks like Kaien. Now keep in mind it's not like they would look exactly alike. Isshin being Kukaku's uncle, would mean that Ichigo and Karen are Kukaku, Ganju and Kaien's cousins, their bloodlines aren't super close
Let me know on what you think about this theory. Do you agree? Do you have any other evidence I might have missed? Let me know your thoughts ^^