我不是妹妹我是姐姐
[wŏ bù shì mèi mèi wŏ shì jiĕ jiĕ]
The user is stating they are not the youngest or innocent (implied by 'not the younger sister'), but rather more mature and possibly protective, as a 'sister' typically conveys. This can imply independence or a wish to be treated with more respect or maturity.