First of all sign them up for a frequent flier account! As a member of a frequent flier chatboard, I have to put that out there :-)
Second and most important...RELAX!!!! If you are a Nervous Nellie around them they will pick up on it and be total wrecks themselves! Don't do that to them. Exhibit confidence and they'll have a great time. Involve them in travel planning--show them the seat map and ask where they want to be (hint: www.seatguru.com will show you the airplane configurations on just about every airline and it'll tell you which seats are good or bad). Pull up airport terminal maps and show them where they'll be going.
Rules for unaccompanied minors are similar for most airlines. The big variation is the youngest age they'll accept as unaccompanied and the restictions on types of flights. At 9 and 11 I think a couple of airlines will allow them to fly on connecting flights but most will only permit direct (ie DEN-JFK with a stop in Dayton but they don't get off the plane) or nonstop flights.
Even though they won't be changing planes, make sure they have diversions (portable DVD player, PSP, GameBoy whatever) to occupy their time while waiting at the gate or worse, if there's an ATC delay and they're stuck on the plane but it's parked at the gate or on the taxiway for an hour (if that's the case, the diversion can't be electronic).
Remember, relax. That goes for your SIL too! Make sure she knows to make the experience pleasant for them. If you do it right, you'll create some traveling monsters and they'll be bugging you to take another airplane trip! There are plenty of bi-coastal/divorced/step kids out there who hold elite frequent flier status.