edit:
@DawnofWar32, family is always a weakness for resistance movements, especially ones againt hostile dictatorships like Nazi Germany. Also added into that is that, unlike the invaded nations, many Germans truly believe in Hitler and can't see him for what he is.
ANY resistance has to be extra careful because of that. Hitler has popular support and an extremely effective secret police force that knows its' home territory far better than the occupied nations it is known for being effective in. Even innocent things like church and scout troops had to be extremely careful with what they did as the Gestapo and other Nazi organisations came down hard on them.