Easiest solution is probably going to be increasing the management/supervision. When your SIL is over, crate your dog with a stuffed kong or chew during meal times, use a gate or exercise pen to keep him out of the kitchen, have him outside with you, teach a settle on mat (see video), etc. Just whatever works for you and your family to prevent any resource guarding from happening.
http://www.youtube.com/watch?v=4uAGaw6KJIc
If you really want to fix it, your SIL will need to actually do much of the work. If willing, have her keep some treats in pockets. Show her the different behaviors your dog knows and have her practice asking for them and rewarding. She should then start asking for those behaviors when she needs your dog to do something. For example, ''come'' to get him out of the kitchen as opposed to pushing him with a foot.
Also, I would start some impulse control training to teach him to be polite rather than try to steal food. You and your mom (doesn't guard from you, correct?) should be the ones to work on this. This is
not something I would have your SIL (or anyone he does guard from) do for now.
I generally start sitting and with food in hand. Back of hand glued to my knee. If you pull your hand up and away, then your dog is likely to jump up to try and get it.
Don't say anything, don't push/pull him away/or try to remove him from the food, just let him figure it out. Keep your hand closed as he licks, paws, etc trying to get the treats. When he stops and backs off, hand opens. If he dives on the food, close fist. If he remains polite, then mark (click or verbal marker if you have one) and/or praise, pick up a treat and give it to him. Continue feeding as long as he remains polite.
Once that is learned, I do different variations...
Treat on knee, treats in a bowl or plate on my lap, plate or bowl on table next to me, treat on floor, etc. Still covering with hand/foot if pushy. Uncover when the dog backs off. If they remains polite pick up and give the treat. Cover anytime they try to steal.
And then I also begin requiring my dogs to actually do more for the rewards. No longer is it, just be polite. It becomes be polite and also do something I ask for in order to get the food. I like to start with a hand target as it means they actually need to move away from food to get the food. Sit, down, tricks, also are great.