Do these characters sound like realistic people? How can I make them more realistic?

I think your characters are great. Physically, the detail is excellent. You should probably explain how they met. I remember seeing a play (it was either a true story or based on a true story) in which a Jewish girl was best friends with a Protestant girl during the Holocaust. The Protestant did a Nazi Youth program and they stopped being friends. By the end of the play, she had quit the program and they were best friends again. It was a sad story, but at least that bit worked out. So you might want to explain how their friendship began, and how it developed into something more. Perhaps Anke's mother was fine with them being friends, but her father was worried that it would develop into more. When it did, both parents were upset - her father was angry and her mother was disappointed. Johannes, on the other hand, could have kept the friendship a secret from his parents. When they found out, Elisabeth was cold and Josef beat him. But these are your characters - feel free to go with my idea, or make something else up. I highly suggest reading the book Writing Magic, by Gail Carson Levine. If you want, just skip to the character questionnaire. It can be very useful for character development. Anyway, you have excellent characters, and it sounds like you've got a great story. Good luck!