Why was Josef the “only adult” in the family now?

Josef’s story begins in Berlin Germany, during the night of Kristallnacht - a true event in history. His father is arrested and taken to Dachau, a concentration camp. After 6 months he is released but only on the condition that he and his family leave the country immediately. They board a train to Hamburg, where they board the MS St. Louis to Cuba. His father has returned to the family unwell, and unstable. Josef celebrates his bar mitzvah on the ship, though his father does not attend, and a funeral. As his father’s paranoia grows deeper, Josef is forced to take on more of a paternalistic role in the family. He has to worry about his father’s health, and watch his sister more often when his mother is staying with his father. After his father jumps overboard, Josef is asked to help with the mutiny, which he agrees to help with, though they are ultimately unsuccessful. Josef and his family are denied entry into Cuba, then the U.S., but are permitted entry into France. Once in France he and his sister enroll in school, but when France is invaded by Germany a few months later, he and his family are on the run again. When the Nazis catch up to them, Josef offers himself instead of his sister, though the reader learns this through Ruthie as Josef dies in a concentration camp.