Ron, Marksmanship is a skill of itself, but asides from that, anyone can kill with a sword just as easily if the intent is there.
Your parents concerns seem to be specificaly regarding your sister's safety. I assume that you have brother/sister rows (its only natural) and I get the impression that maybe these get physical? If that is the case then having weapons easily to hand during such times is a really bad idea.
From now on never strike your sister in arguments, just walk away and calm down. Study your JKD with dedication to prove that martial arts is a genuine passion, not a teenage fad you are going through. I should think that once your actions prove you have mastered your emotions, your parents may compromise with you.