By that logic, you could never prosecute murder.
The reason Chris is charged with incest, and not anything more, is that it requires no victim. The act itself is criminalized. It does require another participant, but the status of victim is irrelevant to the charge.
Given that it seems that Barb is not working with the prosecution, Barb dying would actually worsen Chris' situation, because instead of a potentially defiant Barb, the state would be allowed to write the narrative for how Barb felt without her being around to contradict it.