@ZombieKillerFl, I think if EA had plans on adding wheelchairs to the game, they would have done so with the Get To Work Expansion Pack because of the Doctor career path and hospital that EP adds. Since they didn't add a wheelchair with that EP, then I doubt they will add one in the future. ☹️
While I don't play Custom Content (CC) or Mods in my game because they can be troublesome, there might be some wheelchairs on a site that creates CC & Mods. If I were you, I would search some of them for your sister's sake, since she is upset over this. If even they don't make any, they might take a request since this is an important issue for your sister. I would think though that the animations would be difficult since a wheelchair needs to go everywhere, and there are stairs in the game. But there are some very talented creators out there so it can't hurt to ask! 😇
I hope your sister can find a wheelchair for the game, and if not, then I still hopes she continues to play & enjoy the Sims! ❤️
Best of luck!