The False Princess by Eilis O'Neal

Nalia spends the first 16 years of her life thinking she is the princess and heir to the throne of Thorvald, only to be told that she was only a decoy, switched at birth with the real princess to try to avoid a prophecy made during the queen's pregnancy that the princess would be murdered horribly before her 16th birthday.  The king and queen, having averted their real daughter's death, send the false princess, named Sinda at birth, unceremoniously away from the castle to live in a small village with her only living relative, an aunt who is less than delighted to have a fully grown niece with no domestic skills sprung on her with no means of support.

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