So there is a bacteria that thrives in the warm, moist environment of rice. To be safe, you need to keep the rice's temperature above 60°C or at fridge temperatures (for like three days in the fridge - don't push it if you want to be safe). Rice cookers are supposed to keep rice at this safe temperature for a few hours after the cooking is done. For instance, some Zojirushi models claim theirs are safe for 12 or 20 hours depending on the setting. Theoretically, I think you could keep it longer but it will dry out or overcook which is probably why Zojirushi has this limit. You should check the temperature of your sister's rice to make sure she is being safe. Otherwise, she could get really unlucky one day.

Now as for books, eating them is a bad idea as well but it's harder to find info on that topic. From what I have gathered, paper is mostly cellulose so a little bit is fine but a lot will give you gastrointestinal issues. The dyes are very likely not food-grade. The cover is clearly a tougher material with more dyes, so much more toxic. So don't eat book covers! You can give a page a nibble though maybe.