In my opinion, if you have a genuine reason for making the character German, than you should make her German. In this way I agree with most of the replies you've already received. On the other hand, I think that your mother could have planted a seed in your mind when she suggested that the character could be a Nazi, and because of this you may neglect to include some important aspects of the character's personality and behavior for fear that they may make her seem like a Nazi, which is definitely a real risk that could make the character seem hard to relate to. So in short, change the character if you can, but if you feel that she would improve your story as a German and you can write her that way without making your story seem more repulsive, then by all means, write her as a German.
I find myself urgently wishing that I could present a full-length video of the Fawlty Towers episode "The Germans."
Now I know what will happen, so I'm already hiding in a bomb shelter but ... Fawlty Towers just isn't funny.
I never watch it, that category of sitcom doesn't interest me. But you have to admit as a single clip in this thread it is a little funny.