I think the old bitch deserved to know the truth, is what Yuuko would have wished for, unlike her bitch sister.
Her "repentance" for 60 years, keeping the legend alive and stuff were of course not enough, if she wanted to be fair she should have denounced the murder, made her body get the cerimonial rites many decades before and declared her own part in the murder.
But no.
She decided to live with the shadow of a ghost that wouldn't kill her, gaslighted herself into thinking Yuuko wanted her to suffer for the simple emotion of guilt and not that she was unable to kill her own family, unlike herself.
Don't forget that bitch action made people terify her sister in her last days of life, suffer broken bones, throw in a hole and die of hunger.
ANd then her own father killed himself for letting it happen, AND the thought of living with scum like that.
Hard to say if Yuuko revenge did kill the other bastards or the plague did its job, anyway that was karma.
And again, the ritual didn't fail, they simply didn't made a proper ritual, Yuuko wasn't the most important thing in the bitch life or she would have never sacrificied her, the bitch most important thing in the world was herself.