I have had both hips replaced, and I have had no problems with either.
OK, maybe I'm lucky, but I think it's more that your sis has been unlucky, very unlucky.
Problems with hip replacements are very rare, and what's more, it is possible to rectify the original one, both my mother and my aunt had hip joints re-done.
She should go ahead and get the second one done, and you can go visit, to help her convalesce.
Oh, and another thing, tell her to ask for her car back, sounds like she's being taken advantage of there.