Ok folk I think Yamamura's grandmother is Bourbon.
Why you may ask?
Well I believe that she was black mailed by the black organization, because her grandson got into a gambling problem and couldn't pay it back. So she was forced to join the organization or else they would kill Yamamura. I also believe that Yamamura knows Conan's identity and is acting dumb around him to try to tell Conan that his grandmother is bourbon. Sadly this is one clue Conan has not picked up on. I think that Yamamura's grandmother is closing in on Conan, there have been hints that she kidnapped kogoro and is pretending to be him to watch over Conan.