Keep in mind that to a child this age, possession is ten-tenths of the law. The niceties of ownership and value are irrelevant. If she sees something she likes, she'll have no qualms about taking it. That's normal for a young preschooler or a toddler. (Also, if asked, she'll deny having taken these things if you appear upset.) For the time being, your best offense is a good defense. Keep valuables out of her reach. Instruct her siblings to do the same. And check her backpack frequently. As her brain matures, she'll become better able to understand moral issues such as stealing. That doesn't mean you should stop telling her that it's wrong or that you shouldn't reprimand her. ("I know you want this shiny toy very much, but it belongs to your sister. Give it back.") But don't expect lectures and warnings alone to do the trick at this age.
