There is a sense in which the character of Sophie enteres into a state of denial as she faces the awesome nature of the evil that surrounds her that is embodied in the Third Reich. We need to remember that she herself is in a terrible position, as she is married to a man she does not love and brought up by her father to hold certain views and aspirations. Therefore, in a sense, it is only natural that she should grow up believing in the same kind of attitudes and beleifs as her father and husband.
We can also see her adherence to the tenets of the Third Reich as a defence strategy, too. She believes that bringing up her children to be German rather than anything else will save them, as the Nazis are only focusing on Jews. She definitely responds to her situation by trying to adopt the best policy to ensure her own survival and the survival of her children. However, ironically and tragically, it is her belief in the tenets of the Third Reich that eventually lead to her own suicide as she is unable to reconcile her actions and feels an immense sense of guilt over what happens to her and to her children, and indeed to the Jews as a whole.