Same here, including younger half-sibs who I've never even met, lol. I came to that same realization years ago. I have reached out and even directly asked what I had done to make him not want to talk to me or be around me ever. His response was pretty harsh (basically it was that he had more important things to worry about) but it was an eye-opener, and helped me break loose of the need to try to reach out. Ah, the pain parents can cause their kids - and the pain kids can cause their parents.
I figure that all I can do is accept how other people are, that they are that way and it's not my fault... All we can do as humans is live our lives the way we choose to live them & strive for peace/happiness/whatever our goals are.