There seems to be two schools of thought here. One is that Ran was adopted into the Yakumo early on as a little fox, and Yukari raised her to the point of being a nine-tailed fox.

The other – or probably just me – is that Yukari simply tamed the queen of beasts with her shikigami. Quite possibly, Ran lead the fourth gang in the Animal Realm and decided (poorly) to invade Gensōkyō. She probably did so well, too well that made Yukari decide to give her the brainwash treatment.