Are you hiring a guide for a day of sturgeon fishing? If not, you definitely should as the best result can only be done from a proper boat setup. If you decide to hire a guide, do it early, because most guides are already booked up for September. If you hire a guide, perhaps ask for a salmon/sturgeon combo trip - 4 hours of salmon fishing in the morning (perhaps the Harrison) and 4 hours of sturgeon fishing in the afternoon. Have the trip planned near the beginning of your stay, and get lots of info on fishing spots etc from the guide while you fish. The guide is not only there to find you fish for the day, but is also a walking vault of local fishing knowledge. If you need a recommendation on which guide to go with, let us know.
I still think you guys should plan out a day/overnight trip to the Skagit, or the Thompson (if it is open) while you are here. You can still base in Chilliwack. Find out exactly where to go (from either the guys at Reaction Fly and Tackle on your way to Chilliwack, or from us) before you settle down in Chilliwack to minimize travelling/searching time.
Also keep in mind that the river condition, weather, fish return can be unpredictable, so it's best to bring this thread back up to the top again a couple of weeks before you leave so you can get an update before finalizing your trip plan.