Beyond That, The Sea by Laura Spence-Ash

 Beatrix is growing up in London during WW2. Her parents, Millie and particularly Reg, think it would be safer for her to be living somewhere else till the bombing stops. So they send her off to a family living outside Boston. Mother Nancy has always wanted a daughter so she is thrilled. Her husband Ethan is less enthused about the idea but grows to love Beatrix (or Bea as they call her in America). Younger brother Gerald immediately makes Bea feel welcome while older brother William is cooler at first.

In the beginning, Bea feels like a fish out of water but as time goes on, she begins to feel like this is her real family and where she belongs. Meanwhile back in London, Reg and Millie miss their daughter desperately and it's putting a strain on their marriage. The first half of the book tells about Bea's years with the American family but the second half describes what happens after she goes back "home." It's a compelling story with likeable characters. And you really hope Bea (or as they call her in London "Trixie") figures out where she belongs.