UPDATE:
During a roadtrip this past summer I was driving with my girl, not far from where I was born, and asked what she thought about me trying to find some relatives on my biological father's side, she encouraged me, so we pull over and start searching on phones.
For context, my father has 6 kids (that we know of); In birth order: A daughter 13 years marriage, another daughter by his wife, me (oldest son), a son by his wife, then 2 sons by his sister's best friend.
Sigh... I found out a brother committed suicide in 2018 (his son by his wife), oldest sister moved to Texas, I believe I found my biological father's address, we pulled up, but after second thought I didn't want to meet him, so we pulled off.
I was able to find & meet the second youngest brother, after some tense questions about my identity & intentions he let his guard down and was welcoming. That felt good. I told him I felt like I was kept away intentionally, he replied, "You were. You can't keep someone from their siblings by mistake". That really resonated. We stood outside talking for over an hour, introduced my girl, he's 32, introduced me to 3 of his sons that were outside playing, he has 5 kids total. I didn't get to meet his/our younger brother, he's 30.
Also, visited an aunt and cousin I had met before when I was 18 that was cool, too. They were able to put me in touch with oldest sister in Texas, she's 50, divorced and has 5 kids by her ex-husband that are all young adults now.
We've been keeping in touch periodically.
I still feel like I don't belong, on either side.
It's clear that it was a concerted effort to keep me separated from my father's side. After explaining it to a friend, they called it selfish, by all parties. It's not just the alienation from siblings, but also grandparents when they were still alive, now I have to hear stories and look a pictures of memories that were made without me, ask questions about how my grandmother was, how my grandfather was.
As well as missing a sense of belonging, I'm also missing a sense of identity
"The stone that the builder refused"