usually, we use "kakak" to call everyone who are older than us in Indonesia (1-10 years older than us as long as they are not merried yet). we call them kakak. sometimes we just say "kak". "halo kak. udah lama ga ketemu" (hello bro/sis , long time no see).
kakak can be girl or boy. ( some regions have different way. but generally it can be used to call boy and girl.)
we also call our brother or sister (in our family) use "kakak".
in other regions some people call "abang" , "mas" (elder brother) , mba (elder sister). the meaning is the same.