Maybe some funny socks? Example 1 Example 2 I'm sure there's something more appropriate or meaningful than the ones I've linked.

Audiobooks are also good to pass the time that they can share, especially for your sister if she's mostly stuck at home or in the hospital. "Bossy Pants" by Tina Fey is a hilarious one. I also like Mindy Kaling's and Amy Poehler's autobiographies.