Who is Mimi Rubee from The Murderer's Daughters and what is their importance?

Mimi Rubee is Merry and Lulu's grandmother. She is a fairly young woman when her daughter is murdered by her husband, and as a result, seems to be the reasonable choice for her granddaughters' guardians. Mimi Rubee takes care of the girls for a year before dying of a stroke, which leaves the girls at the mercy of their aunt.