Why is it so important that the Star of David was imprinted on AnneMarie's hand in Chapter 5 of Number the Stars?

The Star of David symbolizes the connection between Annemarie and Ellen and the protection of the Jews.

Annemarie is not Jewish, but her best friend Ellen is.  The Star of David is an important symbol in the Jewish faith, like the cross is to Christianity.  Ellen can’t be seen wearing it because then people will know that she is Jewish.

Ellen is pretending to be Annemarie’s sister.  Annemarie holds the necklace for Ellen.  The Star...

The Star of David symbolizes the connection between Annemarie and Ellen and the protection of the Jews.

Annemarie is not Jewish, but her best friend Ellen is.  The Star of David is an important symbol in the Jewish faith, like the cross is to Christianity.  Ellen can’t be seen wearing it because then people will know that she is Jewish.

Ellen is pretending to be Annemarie’s sister.  Annemarie holds the necklace for Ellen.  The Star of David is clutched in Annemarie’s hand because she is nervous as her family and Ellen are being questioned by the Nazi.  She told him that Ellen was her sister.

Annemarie relaxed the clenched fingers of her right hand, which still clutched Ellen's necklace. She looked down, and saw that she had imprinted the Star of David into her palm. (Ch. 5)

The family is pretty sure that the Nazi did not buy the fact that Ellen is Annemarie’s sister, because Ellen has dark hair and the other girls do not.  This puts her at risk for being identified as Jewish.  The Nazis are collecting the Jews.

The man questioned the fact that Ellen does not look like the other two girls, even after looking at the pictures that Annemarie's father showed him.

He laughed scornfully. "You have a blond child sleeping in the other room. And you have this blond daughter—"He gestured toward Annemarie with his head. "Where did you get the dark haired one? "He twisted the lock of Ellen's hair. "From a different father? From the milkman? (Ch. 5)

The man leaves, but they are not out of the woods yet.  There is a danger that he was suspicious and will come back.

Annemarie’s family is very brave to keep Ellen hidden there.  Her family is part of the underground movement protecting Jews.  The Star of David represents the solidarity between the Christians and Jews.  Annemarie’s family is protecting Ellen’s family from the Nazis.