Eighth Moon is about the twin themes of love and responsibility. "Mama" takes in her sister's daughter and raises the girl as her own, and no one tells Sansan that she has been left behind or that she has another family. Not until she fears she is dying does the grandmother tell Sansan the truth.
Once she learns the truth, Sansan knows that she must go to her "real" mother if she has the chance. She knows that she loves her "real" mother and is sorry that she feels so little responsibility for her adopted mother and father. Her "real" mother feigns illness in Taiwan and Sansan is allowed to go there in order to nurse her back to health. The government acknowledges Sansan's responsibility to her "real" mother so they release her to go and nurse her. Her adopted mother feels (grudgingly) that Sansan has a responsibility to her mother and although she also feels that Sansan owes her respect for having raised her. The grandmother also feels that Sansan should go to her mother.