Reunion Images by Others

I was a step parent adoption and reunited with my first father a few years ago. At first, everyone, except my first father, was on board and excited like I was the missing daughter returned home. One of them, a brother, even knew me when I was little. The reunion eventually did not end well due to his missing a "fatherly feelings chip" coupled with his fear (I think) of "secrets exposure" about why I had been adopted and why he was not in my life as an older child and adult amongst other things that had happened in his life I had privy to. When the reunion failed, he made it his business to falsely represent me to his family so they would break contact with me. None asked me my side, what happened, or anything. I tried to explain and continue relations with them, but I was pretty much cut off based on whatever he lied about. I later discovered some of what was said by him about me and it turned out that he presented a tweaking of events or presented me in a very character damaging way that would make people NOT talk to me. For the life of me, I can't understand why my birth father's family thinks he is blameless for his absence in my life and fall for everything he has said about me currently. I am a professional with a productive life and he has a history of legal trouble and relationship issues. I don't get it. This has stumped me. I have assumed it's due to his wife's support because she was part of some of the tweaking. I am aware of his, and her, history of trouble with one of his other kids. She seems to create trouble involving him on her own. Still, the family believes him and it's all his other kid's fault. Question: Anyone have any idea why a birth parent's family in a step parent adoption believes anything the birth parent says? Any idea as to why a birth parent's family in a step parent adoption would do this and not decide to get to know you IN DEPTH to come to their own conclusions? Especially when he has a history of trouble with a kid he raised?