I felt she'd slightly pulled her punches (or perhaps chosen a less ambitious route) by having the two younger first-person narrative voices. It would have been more compelling, for me, if it had been Aunt Lydia's first-person 'testimony' interspersed perhaps with documentary fragments, scholarly suppositions, etc. about the two younger ones. She's just not as good at writing ingenues so it came over slightly falsely - she's brilliant (and more at home?) at the morally-dubious older women. Basically I wanted it to be more like Alias Grace, I think.

(I also found it compelling in terms of plot/page-turning, though like you there weren't any unexpected twists.)