If she's congested, no to peanut butter, she might not be able to breathe thru her nose. If nauseated, PB is a little heavy. Unless she's diabetic, really fluids are best. They make pedialyte Popsicles.
Try some cinnamon toast & black tea w/ honey. (Just a very light sprinkling of spice, not too much.) A good quality ginger ale helps calm nausea, Coke, too. And I never saw a child turn down a bowl of Cream of Wheat or Campbell's chicken noodle soup. Peanut butter s/b OK, but the high oil content may not help if she's got "the runs".
Yeah, that's fine. Whatever she will eat that will support her immune system and give her some energy. Don't give her any sugar or junk foods. Try peanut butter, broths, yogurts, warm teas. What kind of sick is she? If she has a stomach bug, try putting some acidophiles into her yoghurt. It's flavorless and the good bacteria will heal her tummy and make it feel better.