Personal answer here. I grew up not knowing I had a sibling. The closest thing I had was a female best friend who stayed for weeks at a time with our family. Her home life was difficult as well. Often she would stay for long periods of time. When I went into foster care and tried to keep contact she never wanted to talk about what I had been through. She pretended like it didn't happen. I tried bringing it up and she always ended the conversation. We spent 14 years like sisters. 14 years of her being at my home constantly. Here we are 17 years after me going into foster care and we only speak via facebook. If I have ever REALLY needed her she has always been there for me, but mostly we just don't speak.
I found out I had a brother at 12. Even though he had been adopted by my aunt at 3, he refused to speak of any of the abuse.
Crazy thing is, total there are 3 people (not my brother) I was VERY close to growing up. When I mention my childhood all 3 of them get uncomfortable. Only one just recently in a drunken moment acknowledged what I went through and told me he was sorry it all happened.
No one wants to talk about it unless they can distance themselves from it. When it's close and personal people go into denial. I've come to accept that, and I would love to keep a relationship with those 3 if I could just because it isn't their fault it happened. I understand why they want to deny it. I understand no one signs up for that type of thing. I wish I could regain the friendship I once had with them.
So in short, it ended the friendships mostly, but years later I understand their reactions and wish I could go back and be more understanding of their side of it.