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I've lost this then found this one before, it was on storiesonline.
Kid is hot for his mother, a lawyer, who is strict, and who has an identical twin sister. The twin has been in a mental institution, but comes to stay with them, and seduces the boy having him call her "mom."
She takes a job in the mothers office as like a paralegal, she tells her nephew that shes been in a mental institution because of his mom, she replaces her sister, and they keep her in the basement as if she were the one who had been in an institution, so the aunt replaces the mother.