P.G.
I don't know what to tell you, but I have never looked for my "bio-father". I'm the product of my mother's 1st marriage (out of 4), raised by #2. I never knew the bio, only saw a couple of pictures, thought about it throughtout my older growing up years, but decided against it because he was not a part of my life. I also heard he was not the nicest person (from several people).
I would honor your neice's need to know her bio-dad, but at the same time, she needs to REALLY think about whether or not it is the best thing to meet him. She needs to hear about who he was from people who knew him way back when. She needs to think hard about how it would affect him and HIS family should she put herself in his life. Does she just want to know about him (doesn't interfere in his world), or meet him (MAJOR repurcussions for all).
She's almost an adult, so everyone needs to treat her like one with this situation, and she needs to be guided to think like an adult and not be selfish about it. The need to know is very strong, but she needs to respect herself, her REAL father (who raised her), and her bio-father and his life.
Best of luck with this.