What was Annamarie afraid Kirsti Was going to tell the soldiers?

Annemarie was afraid that Kirsti would wake up and tell the soldiers the truth, or at least something that would allow the soldiers to figure out that Ellen was not a part of the family.

Her parents were standing beside each other, their faces tense, but Kirsti was nowhere in sight. Thank goodness that Kirsti slept through almost everything. If they had wakened her, she would be wailing—or worse, she would be angry, and her fists would fly.

After giving this some thought, I believe your question applies to Chapter Six. If so, the quote below will answer your question.

But Kirsti paid no attention to Mama, as usual. She looked cheerfully at the soldier, and Annemarie knew what she was about to say: This is our friend Ellen and it's her New Year!