Read other letters about this article
I honestly hope this letter is fake.
All in all, I agree with Cary's advice, except this; whether the LW marries the guy or not, she ought to do the decent thing and get the ball rolling on getting this girl some help. Maybe she was horribly spoiled,or traumatized by her childhood, or she has ADHD or aspergers. It is impossible to tell from a letter. Whatever the case, the child has been neglected. Light a fire under the father's ass. Get the child assessed by a professional or professionals. Follow their advice and sign her up for therapy, get her into the recommended programs, whatever. Hell, push the whole damn family into therapy. Whatever is wrong with that child's life will not be solved by foisting her off onto a nanny or shipping her away to school.
She may not love the kid, but I hope the LW has enough humanity to realize that this situation isn't right.