Why is annemarie so confused about the death of great-aunt birte?

Mama and uncle Henrik were speaking in code, discussing the impending holocaust. Annemarie realizes that she never had a great-Aunt Birte. So she realizes she was told a lie and that folks were keeping another danger from her. She was also confused when uncle Henrik said he was going to spend the night on his boat because the weather would be good for fishing. She knew that he never really cared about the weather and that bad weather would not keep him from fishing