From Shotgun Shuffling
Sister X is the mystery sister. Her current age and exact parentage are unknown. It can be assumed her father is Herb since she bears the family birthmark in the form a (you guessed it) x.
First appearance Only I didn't say fudge.
- Sister X's age is unknown.
- Sister X's hair color is a very pale blonde, it's unknown if she dyes it.
- Sister X's is an older sister.
- She uses an upgraded form of Tarraforming, since its previous version causes cancer.
- Indicated by name of this picture that displaced the normal seven Buckingham sisters when she appeared during Black Friday.
- Also confirmed by author comment where he states she is a sister to the other girls and not an aunt and is older than Ginger, she's related through Herb as the birthmarks come through his family, and that either Rosemary isn't her mother or Cinn is dumb/wrong/incapable of basic arithmetic. So a complete toss-up at the moment.
- Sister X's official sisterhood council designation is unknown
- It is still unknown if Sister X will be part of the explanation for why Ellie's IM screen name is The9thSister.
The Author stated that Sister X is actually Tarragon in the future, and that after Anise cut her hair down, her life got into a downward spiral. Juniper also did hurt her eye while Ellie and Cinnamon did something to her too and Tarragon changed her birthmark after that. See here