This is only briefly mentioned in "Two Kinds" but is part of the larger story within The Joy Luck Club. Jing-Mei has grown up in America with her mother, who immigrated from China. In the beginning, we get a glimpse of those whom her mother left behind:
She had come to San Francisco in 1949 after losing everything in China: her mother and father, her home, her first husband, and two daughters, twin baby girls.
Most notably, Jing-Mei's mother has two other daughters whom she was forced to abandon as she fled China. Horrifically, she had to abandon these daughters on the side of a road during the invasion of Japan during World War II.
"Two Kinds" doesn't tell the story of these other daughters, Jing-Mei's half siblings (her mother remarried in America), but from reading The Joy Luck Club, we know that her mother hoped to find these other daughters before she died. She was never able to reconnect with them, and this sense of responsibility falls to Jing-Mei, who seeks to both find her half sisters and to share with them the stories of their mother. Ultimately, this becomes a message of reconciliation in the often broken relationships between mothers and daughters.