What was the single most Earth-shattering thing that has ever happened to you?
I've mentioned mine on here before. I learned ~3 years ago that three of my cousins were actually my half-siblings. My uncle married and he and his wife wanted kids. But it turned out that he had had some disease when he was very young that made him sterile. So they asked my dad (who is the most like my uncle, and was the only one of their brothers who was single at the time) if he would be a donor. He agreed.
Months later, twin girls were born (1986). Then a few years later my aunt/uncle wanted one more kid. So they asked my dad again as well as my mother (his fiancé at the time) if that was okay. They both agreed. A few months later, another set of twins was born (1988). A boy and a girl. Unfortunately the girl passed away at 3mo from SIDS.
Then 2 years after that, I was born. I was raised VERY closely with these cousins (more so than usual cousin relationships tend to be). So when I learned that they were indeed my half-siblings, I was overjoyed and it felt kind of natural. However, it WAS a mindblower. I went to a psychologist later who diagnosed me with PTSD from the shock. Even though it was a "good" shock.
About a year ago, one of the older girls saw on her Ancestry DNA results that my dad was listed as her father. She and the boy approached me to ask if I knew anything. I told them that they should talk to their parents, but that if they had any questions I would answer them.
They have since talked with their parents and have come to their own conclusions on things. I think they're still grappling a little bit with the whole thing, but they seem at peace with it. I've had the opportunity to get closer with them over the past year so that's been nice!