I have almost finished this...but I have guessed what the 'twist' is.
It took me gaes to get into it, because i found the 'Dear Franklin' style too contrived (though once I had guessed the 'twist' that mmade more sense)...all that expositon that would not be in a ltter, no reply...I also found all the main characters unlikeable, her, the dad, Kevin, Celia.
Also, Kevin is so extreme - I don't mean people like him do not exist - that I am sure he would have been picked up by some sort of psych ed service - esp in the v expensive middle class American schools he attends. I don't think the tracks are covered well enough by the dual act he does with his parents or the disguise he does at school, or that the mother didn't ask some kind of outside opinion, despite the dad's reluctance.
Some brilliant bits of prose, but some over-sensationalised structure - cliffhangers etc.
And how is a book about how people sometimes don't get on with Motherhood? Most mothers don't have Kevins!
The nurture / nature debate is a possible goer - except that she takes care to describe Kevin as an aloof, 'difficult' baby right from the start - and it would be a kooky book if we were expected to base K's behaviour on her thoughts during pregnancy.
I think she fell pray t her instincts to write a horror thriller rather than a character based book about mothers!