One possible motif that keeps showing up in the book is family, they all have problems and they all try to get away from those problems with their family. Isabel's family was in Cuba when Castro was the leader and there were a lot of riots so she wanted to escape but she couldn't do it without her family.
Another possible motif is death because every character in the book has to deal with death, Mahmoud lives in Syria in 2015 during the civil war and he has to deal with all of his friends dying and all he can do is hope that he is not one of the deaths.
Another possible motif is worry, every character is worried for example Josef lived in Germany in 1945 and was always worried because that was during World War 2 and Germany was the most hated so he was always worried one of the other nations would take over or because the nazis would force them out because he is Jewish