Hi Michelle,
If your mother-in-law requires some additional assistance or accommodations, I would suggest that upon entering your first theme park, you visit the "Guest Relations" location. At the Magic Kingdom, for example, this is located inside City Hall as you enter the park toward your left. Let the Cast Member who assists you know what her needs and/or concerns are, and he or she may issue you the appropriate Guest Assistance Card. This card would inform the Cast Members who staff the rides and attractions what entrances, seating arrangements, etc. would be best suited for your mother-in-law. For example, in some attractions, sitting in a wheel chair section might help her out. You may also be able to access some attractions via a handicapped entrance.
I know you mentioned that she does not need a wheelchair, but just in case the amount of walking in the parks catches up to her, I've included the link for wheelchair and ECV (Electric Convenience Vehicle) locations and current prices. Feel free to click
HERE if you are interested.
I hope you all have a great trip!