a korean's way of calling a close girl part of their family, much more like an older sister(in a girls view)

A Korean term that means older/big sister that females call a female relative or a older girl they a admire or respect. It is sometimes used as a honorific or to replace a person's name.
There are many variations on spelling like Unni, Eonni, Eonnie, Eunni, Eunnie, or onnie. The male version of this term is Nuna or Noona.