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.

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.)