What is your reason for going? If the purpose of your trip is to spend time with your sister in a fun location, then it won't matter how many rides you cover.
We have many trips to WDW under our belt. One of the smartest things we ever added to our trips was the pre-trip training! About a month before our trips, we take 30min daily walks. WALKING, not running. It might help to know, I've been to WDW with marathon runners who couldn't handle a full day of WDW walking!
It might also help to know that I've also been to WDW with folks who were unable to walk 10 feet.
A big reason to go to WDW is the simple joy of spending time together in a fun location.
Sometimes the highlight of our trip is running to Space Mtn at 1am at the end of 15hours in MK. Sometimes the highlight of our trip is a good meal, or sitting in the lobby of GF while the live band plays classic Disney show tunes. We sing along and watch little girls twirling in princess dresses.
Many folks simply can't imagine what WDW will be like until they go. They don't realize how spread out it is, how much it costs, or they imagine it is all coasters (also common to falsely Universal is all coasters), etc. It is also important to realize that STANDING all day on pavement is VERY taxing on your feet. I've seen all kinds of inappropriate footwear at WDW!
I completely disagree with everyone who says June is better than August. No way! In August, we are acclimated to summer. In May/June, we aren't. Ninety feels MUCH hotter when you aren't acclimated to it. (Winter is the same, but with the cold.)
I say, DO give EVERYONE in your family some pre-trip pointers: pack light; one pair of quality water tolerant, walking shoes, a second pair of quality walking shoes (to swap off), acclimate to the heat, drink water, discuss the costs in detail. Discuss/plan the value of early mornings vs.sleeping late vs. night time entertainment in the parks. Discuss thrill rides vs. shows vs. tame rides. Discuss crowd tolerance.
Last, consider renting a
scooter. You could even rent a scooter with the idea that the two of you can take turns on it! Alternately....some folks save face by pushing a stroller over using a walker, or maybe you just skip some rides, or maybe you don't spend all your time together. Maybe plan (an early) dinner (at your WDW hotel) so sis can easily retreat while other family members return for more rides.