Their Brother Lost in Space

Timothy is adopted by the Snyders, a rich family, and grows up devoted to his three older sisters. Then Matthew, the family's biological son, comes back. He lies to the sisters and works on them until they turn against Timothy, who is cast out and blamed for things he never did. He goes on repaying them anyway. He saves them, and he puts them ahead of his own health. He also volunteers in secret for Project Hail Mary, a mission that sends one person carrying a rare gene out to find a new planet for humanity. Only after he is gone do the Snyders learn what Matthew had been doing and what Timothy gave up for them.