If she is able to go with you, how about a walk and talk session at the park or a nearby mall if the weather is too hot? Help her in the kitchen and share some of what you have learned about healthier eating while you learn some of her family recipes.
If neither of those will fit with your MIL, remember that you likely already incorporate some healthy habits as part of your routine. See if this article can help you check off those you already do or give you ideas for a few more to include while she visits.
Fitting Healthy Habits Into Your Hectic Life
13 Tips to Get More Nutrition and Fitness into Your Day www.sparkpeople.com/resource/wellness_arti