I am not a native English speaker, but "she is a girl of mine" just show, I can't come up with another word right now so this may sounds sexism, possession towards this one specific girl.

However, in the Italian source text, mie ragazze shows clearly that the speaker has many girls, daughters or girlfriends or perhaps he's Hugh Hefner. To me it seems it's important to show that the speaker has more than one girl, which is not stated or implied by your suggested answer.