Next time your parents leave, get her in a headlock and put pressure on her carotid artery, and after about 10 seconds she'll pass out. Then, before she comes to, fit a shock collar around her neck (with a gag over her mouth you can't force oral) and handcuff either arm to opposing bedposts. Then after she wakes up, explain the situation and what you plan on doing to her, and then do it. After you've finished, use a hypodermic needle to administer high-potency heroin parallel to her forearm with the plunger facing the hand. Then untie her (she'll be passed out by this point), and lay her in her room half-naked. When your parents get home, they'll probably send her to a few months of rehab as it's apparent she passed out from heroin overdose and anything she says about what happened between you two will be chalked up as a drug-induced hallucination.
She'll most likely never fight you again.