Ruth's past was very painful to her, and she wanted to forget about it. She was born to a family of Orthodox Jews in Poland, but when she left Virginia to marry her husband, a Christian black man, her family mourned her as if she were dead. That is how they reacted to her marriage outside of the Jewish religion.
Her father was also abusive to her mother, as he had only used his wife to get to America. Tateh, as her father was known, also cheated on his wife and treated his children abusively. Ruth was therefore very motivated to forget about her past. She was elusive speaking about it with her children, in part because she wanted to forget it. In addition, perhaps she feared that the African American community around her would not accept her fully (or her children) if she told them that she was white and was not born Christian.