<human>: You know the drill, I have an older sibling (Sister - 18, I'm a boy - 17) whom basically makes my life a living hell.

At some stages she seems schizophrenic as she acts normally around her friends (Most of whom are my friends as well), a different personality around her boyfriend then around me, she acts like the Antichrist.

From unnecessary taunting to acting like a complete b*tch, there is nothing she wont do to belittle me. I've tried standing up for myself, countering her insults in a mature way (simply calling her immature or pathetic), but somehow she can always turn it back on me and "win" the confrontation.

I would like to know how to "1-up" my older sibling and become the "winner" of any confrontation in a mature way. (In your own experience or your advice.)

Could you distill the essence of the text into 1-3 sentences?
<bot>: Older sister (18) constantly belittling me (Male 17) for no reason, countering insults etc. in a mature way isn't working. Advice needed.