Hiccup has no younger sister in Cressida Cowell’s How to Train Your Dragon book series.
The book version of Hiccup Horrendous Haddock III is the central character in a different continuity from the DreamWorks films. The novels include characters, relationships, and dragon details that do not always match the animated adaptations. Hiccup is presented as the son of Stoick the Vast, but the books do not give him a younger sister named Heather, Valka, or Astrid.
This distinction matters because the films substantially rework the world and its character histories. Valka is Hiccup’s mother in the films, while Heather is a television-series character and Astrid is Hiccup’s friend and later partner in the films. None is Hiccup’s younger sister in the book continuity.
The series consists of twelve main novels published from 2003 to 2015. Its tone, mythology, and character designs differ notably from the better-known animated trilogy.