User blog comment:Miskos3/Fairy Tail Episode 215: The Snake Charmer Ophiuchus, Review/@comment-4489126-20150110050935/@comment-2214433-20150110113416

I am not saying that Yukino learnt it by watching Lucy, but that she was aware of the spell because of Lucy, or she simply knew that Lucy could use it and had a prior knowledge of it. It's basically the same thing as "Oh btw, I studied martial arts", I wouldn't look into it too deep.