The concept of Symmetry is one that is continually referenced through the books. Mathematical symmetry features in the ages of the Huits and the Octos, the year when they were born, and the month and day of their birthdays. Other parallels such as in names are reflective of a greater theory of symmetry.
Based on the fact that it was only when Lucy Huit became pregnant with her ninth child that her sister Queen kidnapped her and her husband, it would seem that this is due to the fact that Lucy was honing a philosophical/mathematical theory of symmetry, using her sister's life and its parallels to her own as an example. Queen was apparently 'jealous', but this seems to mainly manifest as a frustration with her sister's hypocrisy: how could she claim their lives were symmetrical when she was about to have her ninth child?
Though the symmetry of the books often seems implausible, it is thematically interwoven with the inner logic of the stories. The powers that certain members of Lucy's family receive (the females, save Serena) are symmetrical to a certain extent and are likely less specific than their depiction in the series.