Bistro is one of a very small number of WDW restaurants that does not offer a children's menu. The clear intent is to discourage people from bringing children. It's a small, quiet, expensive, restaurant, very different atmosphere from the bustling Chefs de France downstairs.
That said, I don't see the slightest bit of harm in bringing a 12-year-old. I think the concern is more about people bringing toddlers who can't sit through a long, fancy meal without getting restless and loud.
If your sister can't find anything she'd like on the menu, I'll bet they'd be willing to bring her something from the Chefs de France menu. If you want to be sure, just call the restaurant now, and ask.