Reta and Pete M., I don't think either story is likely. Hitler's niece was called Geli (short for Angelika), and she shot herself, as far as is known. As for the prostitute story - see Dale's research of 21 April. I've read a piece by Leip himself where he says just this - that he was linked with both girls, couldn't decide between them and decided to put them both into the song when he wrote it.
Most Germans would still associate the song with Lale Andersen today, I suspect. There is a story that the song virtually saved her life. She fell out of favour with the Nazis for some reason, but they couldn't put her in a concentration camp because she was too well loved as the singer of this song.
Don't know when Marlene took the song up, but certainly not in The Blue Angel. That film was made about seven years before the song.
Pete M., 'Ich hatt einen Kameraden' and 'Die Wacht am Rhein' are much older than the century. They go back at least to the Franco-Prussian War of 1870/71, as far as I know. It seems the Nazis didn't use the former very much. Perhaps it was too elegiac and sad for their purpose. - Susanne