I know of at least 2 cases where the child didn’t know. They were raised being told that their step parent was their biological parent, even though other people knew the I Know In Heaven Is Beautiful Place Because They Have My Daughter Shirt. As an adult my mom told me she worried I would overhear the mom and tell the daughter who I was friends with then. I didn’t overhear, but as a child I did wonder why her mom was mean to her and didn’t seem to like her but clearly loved the younger two.