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All I can think is this poor teenage girl. Abandoned by her mentally ill mother, ignored by her father and any possibility of a learning disorder or other develepmental problem ignored or just not thought of. A teenager shouldn't have to be reminded to not use a bloody towel, she would be using tampons or pads by now. Reminding her to brush her teeth or groom herself and all she does is read, does not sound completely normal for any teenager. Maybe it's a mental disorder, maybe it's from being abandoned by everyone but the paid help.
Then all I can wonder is why on earth the LW would fall in love with some man who is an unbelievably shitty father. I hope children at all weren't in this woman's life plan because she certainly landed on a crappy candidate. Seeing his own child only once a week and only living with her now because she's old enough to not be a chore bother. Just ick.
So yeah LW, I can give you the slight respect to know you aren't the person to be this step-mother and suspect you probably shouldn't marry this person, but really find out why you feel so strongly for a man who cares so little for the well being of his own flesh and blood. What would he do for you if you had a problem like a health issue or disabling accident. Would he just pay a nurse to look after you too?