Kissing Kin by Elswyth Thane

Volume 5 of The Williamsburg Series. This is the adventures of the twins Calvert and Camilla Scott from the First World War through 1934. Both of them go overseas, Camilla to act as nurse's aid in the hospitals run by her cousins in London and Gloucestershire. Calvert to serve briefly on the crew of a big gun. Chiefly it is Camilla's story, her futile love for a Frenchman; her involvement in the stormy passions of Jenny and the American who - with Calvert - had managed to survive the destruction of the gun crew, and who nearly lost his life thereafter. The threads of previous stories are fitted into place, gathering momentum, seeming to build up into a love story between the duke's daughter and the poor mechanic. And in the last quarter, death and disaster; a brief interlude between Camilla and a young Nazi; and the story ends with two matings, and the build-up for World War II.