Time-Travel Series Series by Maiya Williams

Thirteen-year-old Rowan and his eleven-year-old sister Nina, still bereft by the death of their mother the year before, experience an unusual adventure through time when they come to stay with their two eccentric great-aunts in a small town on the Maine coast.

Rowan, Nina, Xanthe, and Xavier have returned to Owatannauk. This time they're travelling on official business, retrieving documents from the famous library at Alexandria, Egypt, before the collection is burnt by Julius Caesar. Xanthe hopes to get a glimpse of Cleopatra.

When Xavier, Xanthe, Nina and Rowan are sent after a time-travel runaway to the California Gold Rush, they're faced with some of their toughest challenges yet: testy gunslingers, greedy prospectors and worse, a society that doesn't treat all of its members equally. But if they work together they might succeed.