When she confronts you about it, deny that you took it-you should practice your innocent face beforehand. You could also pretend to be really offended that she accused you.
[header] How to annoy your sister [title] Steal something from her. [step] One of the best ways to bug your sister is to steal her things. [substeps] Sneak into her room and take something really important, like her ipod, her favorite pair of earrings, or the stuffed animal she sleeps with every night.