I was put in this position as a child by my father and his wife — in different circumstances — with the same "don't tell" rule. After moving out of the family house to be with (and to eventually) marry his secretary, my father and his wife adopted her grandson, Eric, and raised him as their own, a generous act (given that the child's mother wanted to pursue endeavors other than motherhood) that became a dark secret when they forced other family members to keep the fact that their son was adopted confidential. My father's hold over me on this matter was that the few times that he did visit would be taken away, and so because I wanted to see my dad, I kept the secret but never felt right about it during the times I was in Eric's presence.
Volume One: ISBN: 9780996378529:
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