We've been worshipping the wrong brother? That's a new one, for me. But if Isukiri (now that's a nice name for a good Jewish boy) died on the cross (casually?), and what kind of dishonorable older brother did he have to allow him to do it? That's despicable in Japan - and shows the people who came up with it did not understand Christianity.
On the other hand, you could have Jesus appear in a vision to someone in Shinto, post-resurrection, and convert the someone. Unless you want to go with the 12 year stint in Japan studying the divine.....that's it's own explanation for how this AH could've happened.