I’ll explain this one, names changed.
Sarah got pregnant to her boyfriend Joseph in the 1950s. They split up. Sarah had a baby boy.
Sarah went on to marry Andrew, with whom she had another son, my dad. Which makes her eldest son my uncle.
Joseph ended up marrying Sarah’s younger sister, becoming my dad’s uncle.
All of this was basically a secret until the grandparents had died. I was told by a gossipy family member and chose not to spread it.