내가 항상 네곁에 있을거야 is for your same aged/younger friends and your younger siblings( both sister and brother).
언니가 항상 네곁에 있을거야 is for your younger sisters and female friends younger than you.
1 มีนาคม 2015
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You can say that to your yonger sister, but you can say that to your yonger female friends too. (In Korea, people say brother or sister to their close friends.)