Being devil's advocate, Yukariko believed a possibility of sacrifice story, this is a universe where ghosts do exist after all and so might gods. But...
Yukariko clearly was jealous of Yuuko, from her voluptuous body as she gazed on her during bath, her calm mind even during mother funeral, being wise when they were on the mountain trail to her position in family as father were more trusting of her.
Now that she cold blooded murdered Yuuko, her father committed suicide and plague didn't stop at all, was left with nothing but torment and even if she regretted by trying to "save her" for that moment is trying now to desecrate her final resting place...
THE AUDACITY OF THAT BITCH.
Yuuko let her live so she can live a life in regrets and fear of Shadow Yuuko stalking her, deserved but not enough.
I kinda feel Yuuko after regaining her memories will come and forgive Yukariko, but for me she should face the same thing as Yuuko did, breaking Yukariko legs and seal in the cellar.
Thanks for scanlation!