3. After about 1950, if the heroine's sibling dies, the entire book has to be about that. Before about 1950, it could be a sad interlude in a book that's about something else entirely.
4. In the eighties and nineties, the YA novel plot was "Teenager meets dying/mentally ill/emotionally scarred/physically or mentally disabled person/child of said person, and learns the important lesson that you can't fix other people's lives." Thank God, that plot seems to have fallen out of popularity.
Read the comments for a story of brilliant parenting.
Link thanks to yhlee.