Just catching up with this, and it brought back some memories for me. My step daughter is 21 now, and I have raised her since the age of 5. She's always known I wasn't her biological dad, so this issue was dodged somewhat. A few years ago though the dad's sister was on facebook (which is, in my view, the very worst thing to happen to the 21st century) and found my wife. Cue a long rant about how my wife ruined her brother's life, how he was a good boy and is now a good man, husband and father, and all no thanks to my wife, etc. etc. Some years old bad blood came bubbling up.
My step daughter got on to the conversation (not sure how all this is done on facebook). Anyway, she said she was now 17 or 18, whatever the age was, and her biological dad had shown no interest in her whatsoever. never tried to make contact, never sent a card, nothing at all. In fact, he had been on facebook celebrating his "eldest" kid's birthday, ignoring the fact that he had fathered my step daughter. However, if that side of the family wanted to know her, she'd happily meet any of them for a coffee - including her biological father.
Radio silence ensued. After a few more attempts to get in touch with her biological father, my step daughter ended up just shrugging her shoulders at the whole lot of them. They're not interested in her, never will be. She has her own family now - a 1 year old son - and is perfectly happy with the family she has.