In the book, Margrit Balicki is Joseph Balicki's wife. Joseph is the principal of a school in Warsaw. Margrit is the mother of Ruth, Edek, and Bronia.
In the narrative, Margrit Balicki is a loving mother and a courageous woman. When the Nazi soldiers forcibly enter the Balicki home, Margrit is the only one taken away. The text tells us that both Edek and his sisters' bedrooms were locked from the outside. So, neither Edek nor his sisters could come out of their bedrooms while the soldiers were in the house. Although it is never made clear, the implication is that Margrit had locked her children's doors. This is why she was the only one taken away by the soldiers.
The author does not tell us why the soldiers never bothered to break down the doors, as Edek later did with a rifle (to let himself and his sisters out of their bedrooms). However, it is clear that Margrit did what she could to protect her children. Her courage saved her children from being captured.
At the end of the story, we are told that Margrit takes to Jan warmly; presumably an orphan, Jan finds a loving mother in Margrit, who "loved him as much as she loved her own children." So, Margrit can be described as both a loving mother and a courageous woman.