This is a story about data and certainty. It began in century-old genealogy, and ends with contemporary misinformation. Last week I went through a small pile of my grandmother’s papers, specifically those dealing with her attempts, some 90 years ago, at building out her family tree. She was born in 1889, and seems to have been working from some notes that her father wrote about his forebears sometime around 1910. Those jottings touched 1792. Early on, I found out that grandma had a half-sister seven years her senior, from a father whose disappearance went unexplained. I don’t believe this had ever been mentioned to me. The stepsister was adopted by my great-grandfather, but doesn’t seem to have lived with the family. She later married and lived in St. Louis, but when she died aged 22 she was buried in the family’s graveyard in Kansas. If you’re anything like me, you’re already noodling on how all these mysterious fragments might hang together. Who was this person, and what sort of life did she have?…