Who is Sharon Hibbs from Long Time Coming and what is their importance?

Sharon Hibbs was Marnie's sister and David's mother. She was sixteen when she encountered Law, had a summer romance, and ended up pregnant with David. Sharon wasn't interested in David, and Marnie admits that her sister was never a real mother to him. Sharon was a wild child, and when she became pregnant her parents forced her to keep David, saying that she needed to live with the consequences of her actions.

Sharon never returned to school and was never able to hold a job. She was drunk when she was involved in a car wreck that took her life, as well as the lives of two people in another car.