From Shotgun Shuffling
The Seven Sisters
SPOILERS INCLUDED BELOW, AND IN ALL LINKED SISTER PAGES
Daughters of Rosemary and Herb, the Buckingham sisters are a colorful bunch. Each bearing a hereditary birthmark in the form a letter on their cheeks; the letters spell "GUNSHOT" when ordered from youngest to oldest. In addition to the birthmark letters, the sisters share another theme. They were all named after a spice making them... "the spice girls." They typically solve disputes through violence and haircutting, neither of which Quinn was prepared to deal with when Ellie moved in with her.
After introducing them all in flashbacks in Ellie's make-up conversation with Quinn, the author outlined some basic info/stats/description blurbs about all seven sisters in the comment of this comic. Later, at the epilogue to the Black Friday arc, Rusche mentions that sisters 4-6 are basically slightly more flawed repetitions of sisters 1-3 (and Pumpkin is a bonus). Exact statement on the differences is: "Juniper is a less responsible Ginger, Ellie is a lackadaisical Tarra, and Cinnamon is a more hazardous form of ‘fringe sister.’"
"Sister X" made a brief appearance during the Black Friday arc (even chasing the other sisters off their spot on the cast page). She first appeared here, where Rusche also allowed a vote for the readers to choose her backstory without quite knowing what the result of the vote would be. The vote was if Ginger or Tarra would deal with Sister X. Tarra won, which resulted in Sister X being a time-traveling Tarra come back to "tend to the median" starting with Anise ( Juniper, Ellie, and Cinn were to be next). Sister X may also have been motivated to give her past self a warning about a carcinogenic habit. In the author post to for the last comic where we see Sister X, Rusche mentions that Sister X has five differences from Tarra, one brought on by herself, Anise cut her hair, Juniper cost her an eye, Ellie caused her speech bubble to change, and Cinnamon's sin will be shown in good time. Had Ginger won the poll, Sister X would've been their half-sister from a short-lived previous marriage of Herb's and she would not have had a name in the spice motif (which came from Rosemary).
When asked, Rusche posted a screenshot of a spreadsheet showing the bedroom allocations of the Buckingham Sisters growing up. The BG column is not mentioned, but the two guesses are Blind Guy or Bride & Groom to indicate the length of time Herb and Rosemary were married.
Crazed internet denizen TheLastOutlaw likes to believe that the sisters are in some way tied to the Pleiades star cluster because he's insane and is also a huge fan of Celldwellers song "Seven Sisters" and wants to create any excuse to listen to it or have others listen to it.