Complex sentence: She risks her life to help her best friend, Ellen Rosen, by pretending that Ellen is Annemarie's late older sister, who had died earlier in the war as a result of her work with the Danish Resistance.
Split into simpler sentences: She risks her life to help her best friend, Ellen Rosen, by pretending that Ellen is Annemarie's late older sister, who had died earlier in the war. She had been killed as a result of her work with the Danish Resistance.