Just a short break somewhere interesting? A change of scene, a few wanderings, something to take your mind off other people's problems. Don't begrudge the money, it will be money well spent.
As for your sister, I don't know what to suggest if she wants to eat cake and chocolate, can't move now and her husband perhaps likes having her dependent on him. You would think having new hips would inspire her to try to walk, although I have a friend who has put on a lot of weight after a hip operation 