Who is Jackie Mackey from Faithful Place: A Novel and what is their importance?

Jackie is the younger sister of the Mackey, family and the only one of the family in touch with Frank prior to the discovery of Rosie's body. Jackie and Frank meet by accident when she's called on by police for an identification of a suspect. Jackie and Frank spend time together, but Jackie also takes Holly to visit the Mackey family without Frank's knowledge. Jackie is devastated by Kevin's death and then by Shay's arrest.