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It sounds like LW was, herself, somewhat neglected as a child. It is very hard for such people to PARENT, much less step-parent as their resentment of never having been taken care of spills over onto helpless people they are now supposed to take care of.
If the step-parent was raised to be extremely self-reliant ie, denied a childhood, she would be extremely frustrated at the 'easy' things Anna is unable to do for herself.
Neither of you deserves this. Without judging the dad for lack of info, he definitely needs to take Anna to a psych eval.
You, LW, need to be on your own for a while and figure out how to take care of yourself.