The bit where the author is intent upon her being an idiot about believing her sister on that one point where she's obviously savvy enough about literally everything else regarding her family...
I wish this had a little more subtlety. Like if she just acknowledged the possibility that he didn't betray her, but simply wasn't willing to take the risk, that could be understandable, even compelling. It could have real emotional depth and heartache and wistful acceptance of an imperfect world and the possibilities you give up and the good things you find in it anyway, and all such sorts of narratively juicy things.
But this thing going where we're supposed to accept she's insanely competent but suddenly dumb-as-bricks about this one thing? While telegraphing the future melodrama to the audience clear as day? It drives me mad.