The Seven Sisters are legendary minions introduced with v.1.66, each one tied to one of the 6 factions. The seventh card is part of the Neutral card pool.
How to get
Achievement: Each Sister card unlocks via an explicit Achievement when you collect a full set (3x) of six Faction Rares for the respective Sister’s Faction. (or 3x of 6 Neutral Rares for the Neutral Sister)
These Seven Sisters achievement cards cannot be earned with Spirit Orbs, crafted, or disenchanted. Once again, these 7 unique cards can only be collected through the Achievement.