The Samoan word for sister is tuafafine. If you are a male, you would call your sisters, tuafafine, and your brothers, uso. However, if you are female, you would call your sisters, uso, and your brothers, tuagane.
USO is a Samoan word that means brother. The term is gender specific, which means it can be used amongst men and women but never between men and women. Men can call each other USO but Men cannot call women USO.