I have 2 siblings, all three of us are "products of the marriage" however we do not have the same bio father. My brother knows and knows who his bio father is (or who I think it is, several people that have NO CLUE have made similar comments about how much he looks like this person, they were making the comments in innocence ) our sister has no clue. She has been a "open family secret" for years. My brother's parentage has not. I am the genetic offspring of the two people who we all call mom and dad. We look nothing alike.