Hi, Jennifer!  Thanks for bringing your question to the Disney Parks Moms Panel.
I am so excited to hEAR that you will soon have the opportunity to take a 
multi-generational trip to the 
Walt Disney World Resort!  I am sure you will make memories that your entire family will treasure for yEARs to come!  To relive all of those magical moments once you return home, be sure to purchase 
Memory Maker for unlimited downloads of your 
Disney PhotoPass Service pictures.
Unfortunately, all guests ages 10 and up pay the same price for Walt Disney World 
theme park tickets.  Although there is no senior citizen discount, I still think that purchasing theme park admission for your mother-in-law is a great value.  There are numerous attractions in the 
theme parks that are appropriate for all guests, from the youngest baby Mouseketeers to those who are simply young at heart.  The 
list of attractions on the Walt Disney World website can be filtered according to thrill level, interests, age, height, accessibility, and advisories.  This will help you to determine which attractions will be practically perfect in every way for your entire family.
In addition to attractions, the Walt Disney World theme parks offer a wide range of 
entertainment, 
character interactions, 
special events, and 
dining experiences that can be enjoyed by guests of all ages.  If you decide to bring your mother-in-law to the theme parks with you, she will have the opportunity to experience all of these things while creating priceless memories with her family.  That being said, the pools at the 
Walt Disney World Resort Hotels are gorgeous places to relax.  If your mother-in-law decides that the theme parks aren't for her, I'm sure she could have a lovely time sitting poolside with a refreshing beverage, as well.
I hope your entire family has a magical Walt Disney World Resort vacation!  If we can help you with any other aspect of your planning, please visit us again.
See ya real soon!
Samantha