What does papa do to prove to the soldiers that the girls are his daughters?

Papa proved the girls were his daughters by showing them family photographs. Lise had been dark haired.... just like Ellen.

For a moment no one spoke. Then Annemarie, watching in panic, saw her father move swiftly to the small bookcase and take out a book. She saw that he was holding the family photograph album . Very quickly he searched through its pages, found what he was looking for, and tore out three pictures from three separate pages. He handed them to the German officer, who released Ellen's hair. "You will see each of my daughters, each with her name written on the photograph," Papa said.