Oh dear, this is obviously frustrating but your frustration is directed in the wrong direction.

Firstly, although the carriage and delivery of luggage is the responsibility of the operating airline, actually all the work is done by the relevant airports. So if some bags have gone missing you should be angry with the airport baggage handling agents.

When your sister in law discovered their bags had not arrived did they complete a PIR property irregularity report before they left the baggage area? If they did this they should have been given instructions on how to get updates on what is happening with their baggage, this is probably an online system called worldtracer - most airlines use the same system but I have no direct experience of Air China. If you have access to the worldtracer or equivalent system with the PIR number then you usually have visibility of exactly the same information that airline employees would have so normally calling the airline will yield nothing other than frustration as they can't give you any more useful information and it's often an underwhelming experience talking to the luggage department of most airlines. You should also be able to use the worldtracer or online tracing system to give your own updates. So if your sister in law has now moved to a new location give concise updates to say where she is now and how long she will be there.

Another thing to note is there is probably no one looking for your missing bags, at least at this stage. Baggage handlers will periodically sweep their systems and baggage areas for 'exception' baggage that is now where is not where it should be. When they find such baggage the PIR records in worldtracer {or equivalent) are used as the basis of instructions on what to do next. So it's quite usual for the process to take a couple of days.

If your sister in law didn't complete the PIR before they left the baggage area then it's not going to be as easy to sort things out, and this is where you will have to be persistent.

The only other practical measure is to file a missing property report with the airport lost property service for each airport that should have handled the bag, but this is probably something that can wait a few days.

Hope this is useful, good luck in your quest to be reunited with your sister in law's baggage.