Don't Miss Me When I'm Among the Stars

Lydia was born to keep her sister supplied with blood. From childhood the transfusions were scheduled around the sister's needs, and her parents treated her accordingly, at the sister's prompting. Ethan was the one person who was decent to her, and the engagement he offered her turned out to be insurance: the family was afraid that if Lydia married elsewhere she would stop donating. She finds out. After that she cuts all of them off, every family tie and the engagement with it, and puts her energy into training. She becomes the youngest astronaut in the country's history. Her family and Ethan are left on the ground working out what they did, and by the time they come looking for her she has already built a life that has no room in it for them.