Estelle Blofis

Estelle Blofis is the daughter of Paul Blofis and Sally Jackson and the younger half-sister of Percy Jackson.

The Ship of the Dead
While training with Magnus Chase, Percy Jackson mentions that he has to go back to New York and babysit his newborn baby sister.

Trivia

 * Estelle is said to be named after her grandmother despite her grandmother's name being Laura Jackson and not Estelle. It is possible this is actually meant to be Paul's mother, not Sally's.