I actually rather liked it. (Quite enjoyed the book too.) Nothing earth-shattering, but well done with a stylish 60s feel.
I thought the twist-reveal was rather flat, but I did like the way it put the characters into new perspective. Merrikat was rather mopey and unsympathetic, I suppose, but I definitely felt she was the put-upon victim for most of the story. And Cousin Charles, who was a pushy "Mansplaining" douchebag for most of the film, and who lost control at the end--well, maybe, just maybe, he could have been Constance's only chance for happiness.
I can also relate to the story of a once-noble family in decline, wealthy ancestors who were for the most part bastards, and a couple of women clinging to the only life they've ever known well past the time it's time to move on. Merrikat and Constance are a good deal like my mother and grandmother, actually, and sometimes I fret about what my sister might be getting up to all alone in her attick apartment...
RE: Film Revew: 'We Have Always Lived in the Castle' (2019): Messy, Dull