How do the Johansens convince the Nazis that Ellen is part of their family?

The Johansens pass Lise off as their daughter, who was killed before our story began. Like Lise, Ellen was dark haired.... thus, they were even able to pull out a few family photos.

"Lise Margrete,"he read finally ,and stared at Ellen for a long, unwavering moment. In her mind, Annemarie pictured the photograph that he held: the baby, wide-eyed, propped against a pillow, her tiny hand holding a silver teething ring, her barefeet visible below the hem of an embroidered dress. The wispy curls. Dark.