In Astrid Lindgren's The Brothers Lionheart (originally Bröderna Lejonhjärta in Swedish), Jonathan and Karl/Rusky (originally Jonatan and Karl/Skorpan) are two brothers. Jonathan tells his younger, terminally ill, brother, about the fairy-tale land of Nangiyala (originally Nangijala) where people go when they die. It might seem to us like just a story told to comfort a sick child, but it turns out to be true, and the boys go on to have adventures together in Nangiyala.
My question is: how did Jonathan know about what happens to people when they die? Obviously he hadn't died himself before. Is this just a suspend-your-disbelief thing and it doesn't really make sense in an overly pedantic reading of the story, or is there some in-universe explanation?