Hello, Karen!
Unfortunately, the Wild Africa Trek has a weight limit of 300 pounds; my Moms Panel buddy Kirsten just took this Trek last week and tells me that guides weigh Guests at the beginning of the tour, in order to determine harness size, so you'd immediately discover that your sister cannot participate. Furthermore, Guests on this tour should be prepared for a three-hour "outdoor physical journey," so this may not be the best choice for her trip to Disney's Animal Kingdom.
However, there is an "Alternate Tour," which is less intense physically. I suggest that you contact Cast Members at Wild Africa Trek at (407) 938-1373 and explain your sister's interests and needs; they'll be able to advise you about the possibility of the Alternate Tour better than anyone! The office operating hours, by the way, are from 9:30 until 4:00 p.m., seven days a week.
Should you determine that you are not comfortable with the Alternate Tour either, consider a milder tour such as
Backstage Safari or
Wild By Design.
Have a wonderful visit to Walt Disney World! Please be sure to send additional questions as you think of them! I'd love to help you plan the most magical vacation for you and your sister!