What's the best Prank I could prank on my sister without getting in too much trouble?

My sister is having a sleepover in her room with one friend and my sister is 10 with the guest 12. I'm having a friend over and were 11 and 12 and hopefully it could be a two-man prank. We need something where they won't get so freaked out that they tattle-tale like always and not get in much trouble at all if they do.