The Color of Water is a memoir about a family comprised of two distinct and relatively segregated cultures. Ruth, the author's mother, was raised as on Orthodox Jew. Her family kept the Sabbath traditions and rituals faithfully, were educated in Jewish schul as well as public school, and ate kosher. Her parents wanted her to marry a Jewish boy, and her father went to great pains to set her up with anyone Jewish (much to Ruth's dismay). Instead, she chose to leave home and marry a black man.
Once Ruth does this, she is dead to her family. When Ruth asks to visit her dying mother in chapter 21 of the novel, she is told the family has sat shiva for her--the Jewish rite of mourning. It involves praying, turning mirrors down, covering their heads, and sitting on boxes for seven days. While she is not actually dead, once they sit shivafor her, she is dead to them. That means Ruth cannot come visit her mother, even though her Mameh is dying.