Tell a girl that you, her, and a couple of friends are going camping for the weekend. Then assure her that some crazy parties are gonna happen at the camp and if her parents know they definately won't let her go for sure. Then convince her to tell her parents that she is going to sleep over at a girlfriend's house. With that accomplished pick her up and a couple of friends and head over to the camp grounds.
From her just start yelling at her all stupid for no reason and tie her up and put her in a log cabin and tell her that you're gonna kill her. Then remind her that no one will be able to find her body because she told nobody else that she was going camping with you and her parents think that she's sleeping over a girlfriend's house. Then laugh maniacally and have a deranged look on your face. Your friends should go up to her face and laugh too by the way. Then put her in the back of your trunk and from here you have two options.
1) Take her to a nearby lake: Take her out of the trunk and then say that you're gonna put her in a bag with a pile of bricks and say she will suffocate on the blue waters of the dark abyss. Say that phrase too because then she'll know you're insane for sure.
2) Take her to a heavily densed forest: Plan to shoot her execution style with your dad's old shotgun. Put a blind fold on her too so she doesn't know when it's coming. Making a couple of just trigger sounds so she knows that you have a gun and could kill her at any moment. Then say that she shouldn't have take some of your popcorn at the movie theatre that one time five years ago and this was payback. That should convince her you're insane for sure.
Doesn't matter which one you choose they're both pretty good and then at the last moment after you say "We coulda been good together! I loved you baby!" and she'll be all confused because she's your sister, then say APRIL FOOLS. All your friends will laugh seriously. It'll be sooooooooo funny. Then untie her and reassure her its a joke. She'll laugh too even though she still might call the cops and file a restraining order but still that dumb bitch could never take a joke.
Here's a video that shows it:
http://www.thatvideosite.com/video/2937 (These guys are fucked up in the head!)