I'd like to give this the perspective of the girl who doesn't know who her real dad is. About a year ago, I found out the man whom I've always thought was my dad (who actually died when I was 1 year old) was, in fact, not my father. I found out my "real dad's" name, but with all the searching I could manage, I've never found him. Personally, it was absolutely devastating to me. The situation was different as my mom intentionally lied to me and I never had a living father. Even now she has only threatened me about trying to find the man. I no longer have any relationship with my mother and this incident was the final straw. I have to say that not knowing, then finding out suddenly is an AWFUL feeling. This girl has the advantage of having a man who did love her and take care of her. I think if I had a "father" the information that he wasn't my biological father wouldn't be as painful. But me having my illusion of a father taken away and knowing I lived my entire life without a living father, only to find out one was alive was reality shattering, to say the least. Either way, if you tell this girl, its going to cause her significant pain. Especially considering that your ex so easily gave up his parental rights, aside from finding out she has siblings, she is going to find out that her bio father "didn't want her". I have no advice either way to whether you should tell her, but think about what I've said. You are going to create a happy situation for your son, while you could actually be causing a great bit of destruction in her life.