If she says going on the computer fries your brain, then tell her "so you can use it without having your brain fried?" Maybe you should just have her be alone for a REALLY long time...or have you and your dad talk to her. And if she tries to avoid it, just use good ol` duct tape to cover the mouth and restrain the arms.
You've got it tough. Annoying could be worse than abusive, since they're not hurting you, so whatever you do can't really be proved or justified.