my mom is one of 6 kids, 3 girls, 3 boys. when her father died, it came to light that prior to their family, he had a daughter. all of the "kids" were adults in their 40s/early 50s when this came out. the 3 girls embraced their new sister (she was found not long after, lived in a different state) and have developed a relationship with her and her children. the 3 boys, on the other hand, have wanted nothing to do with her at all. no interest. not sure why the divide, but I do find it interesting how everyone reacted to the news.
my dh has 3 half brothers. actually, only 2 now, because one was in a tragic accident a few years ago. they don't know of dh's existence. dh's father has never had anything to do with him. mom was young (19) when she had him, they weren't married, and from what I understand dad just wasn't interested/around after she found out she was pg. dh had a brief meeting with him right before we were married, and dh said he was sooooo excited to meet dh, and confessed to never telling his wife/kids about him. said he was going to go home and tell the wife/kids about him that week, and was excited to get to know dh and all that jazz. dh never heard from him. I often wonder if they boys (now men) will discover about dh and try to find him someday, but based on my uncles reaction to their half sibling, I guess I doubt it now. I think I have it in my mind that women would be more interested in men in finding/developing a relationship like this. granted, I also know I am going only by my own, very limited, experience here. so who knows!