Coincidence is one of the classics of fiction. And a common coincidence is that person X is really an unknown blood relation of Y (someone very different) -- with that connection discovered at some dramatic moment.
Of course, JB has used this already with Thomas, but I think there is another good prospect that has not been explored: Kumori. All the speculation around Kumori is around "which female character introduced in the series is really Kumori". I have to admit that none of the theories seemed very probable to me. I agree that there is some connection there, but I think we are asking the wrong question. I want to change the question to: "Who among the people Harry knows is Kumori related to? "
When you ask the question that way, you get an easy answer loaded with all sorts of strong emotional connections with Dresden and many of Dresden's allies. Well you get two answers.
1) Dresden's older sister would obviously have the most powerful effect on Dresden himself. We also have had some foreshadowing that other siblings might exist. This is not my favorite simply because it was done before with Thomas. JB does not repeat things often.
2) Charity has a powerful magical bloodline. If she had a sister or close cousin who did receive magical training (aka - Kumori), that would also create a very powerful connection to Harry, Michael, Charity, Molly/Winter Lady, and indirectly to all the other human allies of Dresden. It is a connection that the Outsiders would have every incentive to leverage against Dresden. It is also a connection that is likely to blindside Harry simply because he trusts the Carpenters so much. This idea has possibilities!
