Your Time or Mine Forever: Book 4 of quadrilogy 'When Times Collide'

Sylvia Danbury and Madeleine Robbins are shot into 1914 as the passage Sylvia uses to go from her time to the past where Madeleine lives becomes unstable. Madeleine, who had not previously travelled in time, has followed Sylvia into the passage and there is no way to return to their times. They take on the identities in 1914 of people they know in 1948 who look somewhat like them. Madeleine becomes Katherine Nouveau a French singer and Denise Nightingale a nursing sister. Sylvia becomes Nyree Nouveau, a French singer and the sister of Katherine, and Ann Lombard, a doctor.
The 1914-1918 war progresses and they are caught up in war work and the housing of refugees as they move to Switzerland.