Sad they don't change the format or card rules a bit. Limits the cards able to be used. They already have this with Dark Magician. Just make it so only Harpie Lady Sisters can use it or if you have 3 'Harpie Ladies' On the field. I know this idea isn't perfect but still. It really would be no different than Dark Magician spell Dark Magic Attack.
There's a couple that work differently to how they do in the TCG, even though they use the TCG wording. (Hallowed Life Barrier is a good example of this.) Not relevant in this case, but something to keep in mind.