A lot of it (maybe most) is true
Reading through these reviews, I'm struck by how many readers don't know that the book is closely based on GSP's own childhood in Indiana. Laddie is her older brother Leander (whom everyone called Laddie), and her brother Leon and sister May really existed (their real names were Lemon and Ada), as did all of her 11 older siblings. Many episodes in the book, including Little Sister's "hanging" by Leon, Bobbie the rooster who crowed to end a prayer, and Leon's targeting of the church congregation with aptly-chosen Bible verses really happened, as documented by GSPs biographer Judith Reich Long. I think only the more dramatic sections, such as the revelation of the Pryors' secret by Shelley's beau, were fiction. Sadly, Laddie drowned at the age of 19 while swimming in the Wabash river. Per Ms. Long's book, some years after the publication of "Laddie" in 1913 GSP responded to a letter from a reader inquiring about what happened to the Princess, saying that many years after Laddie's death she was at last able to make a happy marriage, and that she enjoyed a long life.