These sisters seem to despawn at 4am game time. I was wondering around in BC saw these sisters up. As I got ready to pull this one she despawned. I started to get my next one and she despawned as well.
Her knockback is about 20 meters. Its a normal type that interrupts spells and whatnot but not much else. If your soloing her, she quickly catches up to you in your new location.
Pirate Skull Ring is her common drop. Neither of her two loots have anything special about them like the other sister's loots.
The main reason to kill her is to get the others to spawn or if you are doing the kill-all-four quest.