Who is Third Sister-in-Law from Snow Flower and the Secret Fan and what is their importance?

In the Lu household, this woman is married to one of the Lu sons and is the mother of his children. Her husband and all of her children die from typhoid. Madame Lu spits at her and slaps her and hopes for her painful death, since she is in such deep grief over losing her son and grandchildren. Third Sister-in-Law drinks lye and kills herself.