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If the LW marries this guy, she will have to be a mom to the clueless, irresponsible dad as well as to the daughter. And I agree with other posters that if the LW does nothing else, she should encourage dad to have daughter checked out by a psychologist and a doctor. It sounds like the girl has developmental difficulties and dad can't or won't face up to that. Helping them both would be the right thing to do.