Of Stars and Secrets by Michele Venne

Secrets can instigate peril, or save her life.
Kiersten abandoned the Prince of Dulsar before they fulfilled their marriage contract in order to continue delivering the vaccine to combat a virus decimating newly colonized planets. She encounters Aiden on Station 5 where he joins in her to assist in the release of her brother from incarceration, before she returns to Suma and her royal duty. They must decide if their discoveries will save lives and join the most powerful planets in an effort to advance the Alliance.