So Shiki and Rei are still sisters, with both still being the Shiyuu Ritual trainees for their tribe.
Shiki is exceptionally talented and is basically expected to be the future Shiyuu by her clan. In comparison to her sister and in general, Rei sucks at doing the Priestess Dance. Let's make things interesting and say that she cannot even do the Dance for whatever reason. Lol.
So Rei cannot understand why she is a candidate at all and no amount of her bitching ever dissuades the Elders, with the official explanation being "Tradition dictates siblings, yadda yadda, your sister is great so she is our chosen candidate, sure sucks to be you" and the actual reason being, you know, the whole "We want candidates to kill their sister thus severing all emotional attachments" deal.
Obviously Rei feels inadequate compared to her sister but Rei refuses to simply count the days to her supposed inevitable death, so she looks for alternative ways to fight. Camouflaged clothes under her robes, hidden blades, throwing weapons, poison, flammable oil/powder, blowing darts, pocket sand, you name it and she has it.
Despite the difference in treatment from their Elders, Shiki and Rei remain steadfast friends (as the Elders still want them to be), with Shiki even looking up to Rei due to the latter's strength of character, creativity and refusal to stay down.
Years later, come the day of the Ritual, both sisters kill numerous candidates with Shiki being an engine of gory destruction with her perfected Shiyuu Dance and Rei silently hunting down both the weak and weakened with extreme prejudice.
Soon, only the sisters are left in what each considers the worst case scenario as they both hoped that they would not be forced to face the other. Rei is especially in a worst case scenario since she had some difficulty with finishing off another candidate which lead to her being exposed to Shiki.
Rei wholeheartedly believes she is screwed. She is out of tricks, her camo is useless and Shiki is way too close for her to run with both being in striking reach of the other's blade.
All Shiki has to do is cut her down in a second. Despite all this, Rei still refuses to die and strikes wildly at Shiki anyway. Knowing the futility of her last action, she closes her eyes as she fully expects to die.
A slight but sharp pain in her neck is followed by a splash of blood on her face.
Opening her eyes, Rei finds her sister impaled on her blade with Shiki's sword digging ever so minutely into her neck.
No words are spoken but as the light leaves Shiki's eyes and her blade falls to the ground, it dawns on everyone present what has transpired.
Shiki, for all of her power, could not kill her beloved sister. Rei, despite her lack of power, was willing to bury her sibling with her own hands.
Rei is shocked and distraught but alive. The Elders are livid that their groomed candidate that was so extraordinarily talented ultimately never possessed the mindset to be a true Shiyuu while the candidate whose only purpose was to die by her sister's hand and whose unconventional fighting style completely ignored all tradition, ended up completing all aspects of the ritual.
Of course, someone is dumb enough to say that last part out loud which snaps Rei out of her misery and quickly turns her grief into apoplectic rage as she realises both the entire point of the Ritual and the reason why despite her obvious lack of talent, she was forced to compete.
Rei knows that death alone will neither scare the Elders nor make them suffer to their core but she knows what will.
She knows that the only thing that the Elders have ever cared for is the creation of the Shiyuu after each generation. Thus she makes her intention clear to the Elders that to make them pay, she will wipe out the ability for another Shiyuu to ever be created again. She will take everything from them as they took everything from her.
The Elders are horrified at the notion but believe that Rei is all talk. Rei escapes to heal her wounds and gather more supplies before embarking on a months long campaign against the tribes, assassinating the Elders and any others who could teach the next generation of Shiyuu the Priestess Dance.
Her job is made easier by the fact that only a few years ago, Kyoukai decimated the ranks of the Shiyuu Tribes in her rampage against Yuuren. As such, there is little opposition to her actions.
As she makes good on her word and kills the last of the Elders, she is left hollow, her anger diminished as the weight of events, especially her sister's death, catch up with her.
She tries to think of anything to distract her from her mental torture and she remembers Kyou Kai. Kai, who killed the previous Shiyuu thus resulting in the latest Ritual that her sister died in. Kai, who is now the last Priestess Dancer, the last remnant of the Elder's work. Kai, the only person who could possibly pass on the knowledge to make another Shiyuu. Kai, as Rei convinces herself, who must die.
Thus Rei leaves on a vengeful quest fuelled by misplaced hatred and deep down, a desire to lose herself in something, anything, that distracts her from her act of siblicide.
Ultimately, she has achieved her lifelong objective of staying alive but in a way that has left her rejecting the act of truly living.
From here, she picks up more underhanded methods of dealing with foes, always she seeks new ways to kill opponents in unexpected ways in preparation for her inevitable fight with Kai.
