Put something in her bed. The old cliché is a snake, of course, but it’s cliché mainly because it works. Got with a rubber snake, rather than a live one, and nestle it where her feet are going to end up. You can also place it in or around her pillow.
Tap on her bedroom window. Sneak outside her window in the middle of the night and just start tapping. If she likes to stay up late in the living room to watch TV, you can choose to tap on that window instead. You can also bring a flashlight to shine in the window while you tap to add to the terror. Make sure to call it quits before she gets too scared, and let her know that it was you. You don't want to end up actually hurt from her confusing with with a prowler. Keep this trick short and simple.
Mess with the bathroom scale. Women quite often get obsessed with their weight. If your sister is old enough to be in this category, adjust the bathroom scale to add 10 pounds over the real weight. She will be screaming from the bathroom after this one.
Hide her favorite outfit. Fashion is another focus for many a sister, and her whole world will crumble in hideous terror if her favorite outfit has gone missing. This will be especially effective when coupled with the tapping, as she’s prone to think someone broke in during the night to steal her outfit.
Place something terrifying in her hair gel, breakfast cereal or shoes. Anywhere she doesn’t expect to find a rubber eyeball, giant bug, rubber rat or severed rubber finger will get the adrenaline pumping.