I think when the girls get together, all four of them something could happen. Maybe they have a welcome home BBQ and before it starts the demon(s) attack causing confusion. The back garden will be all set up. This is where they learn they are witches as there powers stop being bind by the spell placed on them from when they were young. At first they will just be dodge until one who is in danger uses a power by mistake, most like the freezing one which leads them going to the attic where the mother explain who they are and passes them the book of shadows where the charmed ones become active. A random attack on a random day, I will introduce a time skip to get to that point.
Thoughts?
Thoughts?