Sister and Kikuchi???
I'm not looking definite answers but why did he shoot his sister?
Was it Kikuchi, the rookie, who replaced the Iwaki, the fried cop, as corrupt Yakuza connection??
He shot his sister because she had become a heroin addict and in his psychopathic mind death was a better place to be for her.
As for Kikuchi he might not have been corrupt the whole way through but at the end he is willing to take the Bribes that Takeshi wouldn't.
What was wrong with her was never specified probably he felt it was some mercy killing given his tendencies, what had been leading up to what he is and other factors led him to do it. Through frankly I can't reconcile it myself.
As for Kikuchi well I found it very sad he couldn't have learned from the mistakes of Azuma and Iwaki.