found out my brother and I have different fathers
I (23F) have two siblings, Sam (16M) and James (12M). Some details have been changed for anonymity.
Our parents have been divorced for 5 years. Our dad was having an affair with his co-worker. While it was a clean break, I had just turned 18 back then and was going out of state for university. It was extremely hard for me as the eldest sister to carry the weight of this, trying to support my siblings while also figuring out adulthood.
It's been years, I don't go to our small town often because I went into corporate as a fresh graduate. My siblings are both doing good at school as well. I recently went back home as we've been planning to stay at our maternal grandparents' place for a small get together. Since I don't usually get the chance to, I also went out with childhood friends around town. One of them, Lily (26F), is my cousin from my dad's side.
Lily and I have always gotten along, so we were talking about everything from the past to how we were doing now. When it got to the topic of my parent's divorce, Lily said she really commended how well I handled everything. She kept going on and on about this, it was pretty awkward for me as I, at that point, believed that I've already processed it as an adult and it's the past anyway. I told her as much and that I even went to therapy. Lily then said "if I found out about my sibling I wouldn't have been able accept it" (not verbatim). Of course, I was extremely confused and asked her what she meant. She was surprised and proceeded to tell me that my father's side of the family always suspected that James is not his child. Their talks during the divorce basically confirmed it.
I never knew about this until now. Obviously, I don't give a shit. He's my baby brother, I grew up with him as my sibling, and he'll always be family to me no matter the technicalities.
About Sam and James, I've decided I'll wait until they're both old enough to tell them because I don't want to keep this secret from them forever. But I also don't want this to affect us right now as they're both in a good place and it won't change anything about our relationship.