The Oscars are coming, and Richard Linklater’s “Boyhood,” already a critical favorite, is a contender for many of the big awards. As the title declares, the film is very much a boy’s coming-of-age story, but “Boyhood” is also about girlhood. Mason has a sister, Samantha, who grows up alongside him over the course of the 12 years it took to make the film.

For the first half of the film, as Mason dreams, Samantha competes with him. She dominates, teases and outperforms her younger brother (in reality, the actors playing the...