Kinda? [She looked a little sheepish about that.] She was the one that actually got into the attic and found our Book of Shadows. It was in there that she read the spell that unlocked our powers.
But, beyond that, we needed her because our powers are linked. We're what magical prophecies foresaw as the Charmed Ones. We have the Power of Three, and it needs three sisters to work. If Phoebe hadn't come back, I don't think we could have unlocked our powers at all.