Who is Miriam from Daniel Half Human and what is their importance?

Miriam is Sebastian's daughter, Sophie's brother, and Daniel's cousin. She intends to move to Palestine as part of the Zionist movement before her father is imprisoned for his socialist activities, after which, Miriam comes to live with Daniel's family.

Sophie remains in Germant with her father in 1939 as Daniel and his family are leaving the country. Her ultimate fate is unknown, but Daniel later wonders if she was eventually sent to a concentration camp.