Sorry, I have had to go anon as people on here know me IRL and I don't want any chance of this getting out.
My niece is 17. My sister met her now ex husband when my niece was just under a year old. My niece has always called this man Dad and my sister had his name put on the birth certificate as my nieces father, although when she registered the birth she put 'father unknown'. My niece has no idea that her 'Dad' is not her Dad and my sister has no intention of ever telling her.
However, recently my sisters ex husband has said that he feels they should tell their daughter he is not their Dad. He is a decent man and although he and my sister also have a son, he treats both children in the same way and seems to feel no differently about my niece despite her not being his biological child.
I personally feel that it is unfair to never tell my niece that she has another Dad, but my sister really insists she is never going to tell her, despite the fact my niece has already said she thinks her Dad isn't her Dad. (the extended family all know and I suspect she may have overheard conversations maybe?)
I hope all this makes sense, but obviously it's not my decision to make at all. I just wondered what other's take on this situation would be?