Ruby Red by Kerstin Gier

1st in Ruby Red series.  Sequel is Sapphire Blue.  First published in Germany.

Gwen is 16 and lives with her mum, siblings, grandmother, aunt and cousin in an upperclass London neighborhood.  Her extended family doesn't pay much attention to her, focused on her cousin, believed to be the carrier of the time-travel gene that runs through her family and sure to show up very soon in the form of an abrupt trip to somewhere in the past.  But all is not as it seems.  Since a mathematical formula had worked out that Gwen's real birth  date was the birth date of the next time traveler, and her cousin was also born that day, Gwen's mum and (late) father secretly falsified her records to indicate Gwen was born the day before, allegedly in the hopes that she would be able to live a normal life (and since she was born at home with a midwife, no one was the wiser.

Crossed, by Allie Condie

Sequel to Matched.

Nathaniel Fludd, Beastologist by R.L. LaFevers

Book 1: Flight of the Phoenix

When his parents disappeared while off adventuring, Nathaniel is sent to live with Phil A. Fludd, and aunt he did not know. With her he discovers that his parents, aunt and most of his ancestors are beastologists, "someone who studies beasts. Not any old beast, mind you. Only unusual beasts...like dodos. Or basilisks, or griffins, or manticores and the like." But Nate isn't sure he's cut out to be an explorer or adventurer, especially when his first beastology trip with his aunt results in her being held prisoner by Bedouins and Nate on his own to make sure a phoenix egg hatches, figure out how to rescue his aunt and keep the families Book of Beasts safe from a mysterious man who wants to steal it.

Sequel: The Basilisk's Lair

SLJ grades 3-5