The Stillman Sisters were a trio of witches who used dark magic against the Charmed Ones in Charmed.
The Stillman sisters were witches but abused their powers by willfully using dark magic for crimes; they were known for committing magical crimes and were wanted in several states such as California.
In the episode they are introduced as the Charmed Ones breaking into the attic to steal the Book of Shadows but it scoots away from them. They are Margo, Mitzy and Mabel. Mitzy and Margo go on about the plan not working but Mabel says that things will always work out. First they need to take on the identities of the Charmed Ones. Then steal the Book. They hear Paige coming in so they teleport out of the house.
Later, the sisters move into a house across from the Charmed Ones, killing the inhabitants. They then view the Halliwell sisters through a scrying bowl and Margo says how she wants to throw Wyatt, Paige's son, into a volcano.
Afterwards Mitzy goes on about how she wants Phoebe's white-lighter powers and how she never gets anything good. She says she wants the best powers and how Piper's powers will suck. Mitzy and Margo get into a fight but then Margo comes to break it up by killing a salesman who came to sell them a new hoover, and reminds her sisters that once they have the Book they'll be all powerful.
In the afternoon they perform the dark spell to steal the identities of the Halliwells, which goes something like this:
Blinking faces, bland and ho hum
We are they and they are no one
Grant to us the power of three
And turn them into nobody.
To prove it worked, Margo summons Chris the white-lighter who has to do the Halliwell's bidding. He thinks the Stillmans are the Halliwells. This delights the evil witches.
The Stillmans tell Chris to orb to the Astral Plane because of a knife they used to kill the salesman but Margo pretends a demon did it.
Chris later finds out they lied to him but they try and seduce him but it doesn't work. Chris begins to suspect they may not be the Halliwells. He finds the Halliwells who have been kicked out of their jobs after their ID cards showed the Stillman sisters as their employees instead.
Chris manages to get the Halliwells to their house where the Stillmans are poring over the Book to get an entry. They do, but its a very small entry. It just says use a binding spell to imprison the sisters, not kill them because they are unworthy of being killed. The Stillmans are offended by this.
The Halliwells show up and fight the Stillmans but the Stillmans win and Mitzy gets angry when she can't orb but Phoebe can. Later the Halliwells meet the Stillmans and have a showdown when they manage to make Margo think Mitzy hates her and make the sisters fight. The Halliwells subdue the Stillmans and get back their powers. Then they turn the Stillmans over to police for murder.