Who is Marnie's Mother from Long Time Coming and what is their importance?

Marnie's mother sends Law letters from her current location in the nursing home. She is dying. The nurses are happy that she is writing letters, but Marnie quickly figures out that the letters are to Law. When Marnie questions her mother about the letters, she notes that the older woman was filled with spite. Marnie's mother believed that Law should be forced to take responsibility for David, as she and her family had done for David's entire life. It's noteworthy that Marnie's mother must have known the level of devotion and love Marnie holds for David. She also must have known that it's possible that Law would demand custody. Her motives are never made any clearer than her desire to force Law into the role of responsibility, but it seems possible that she might also have been worried about her daughter, who would now be left alone with a teenage son to raise.