If you don't have a solution already, here is what you need to do:
Visit your router's admin settings. You can do this by opening up a web browser of your choice and typing
192.168.1.1into your url bar.
Now, you should be presented with a login menu. Depending on your parents and your router, you might be able to get in. If your parent's changed the default password, you're more or less screwed unless you want to factory reset the router by pressing the reset button on the router. However, if you do this, all the settings, including the WEP password and other configured settings, will be reset and your parents would find out unless you can set these after.
If your parents have not changed the default user and pass, you can just type that in and gain access. There are plenty of lists of default passwords on the internet if you simply type in your router brand and model into google. Most linksys routers leave the user blank and the password is "admin". If you think you have the right info, don't be discouraged if you can't get in right away. A few routers require you to login in correctly multiple times as an extra countermeasure for security.
Once you're in, go to the Accessibility page. All router GUIs are different, so I can't really direct you step by step, but you need to enable MAC filtering. Before you can proceed, you need to find her MAC address. Open command prompt on her computer (I'm assuming it's a PC, if she as a Mac, I don't know the exact procedure) by click Start > Run > CMD. The command prompt will appear. Now you need to type
ipconfig /alland look under the active adapter. There should be a thing called "Physical Address" followed by a 12 character code in hexadecimal format (ie XX-XX-XX-XX-XX-XX). Write this number down. Go back to your computer and set the filter to block listed addresses. Enter in her address and then save the config. Whenever you want to kick her off the internet, enable the filter. When you're done, disable it.
Though, if you have access to her computer, you could always just disable her NIC and then enable it when you're done.
