Mabel is the sister of Kelsey Louise. Kelsey says that her sister is waiting for her at Aunt's house, and will insist that Nick takes her there during the side mission Kin & Comfort. The family was meeting up in Los Perdidos for a family reunion.
Nick will escort Kelsey to the family house where she states that the family would be very grateful for Nick's help. Unfortunately Kelsey will find her family zombified in the dining room by unknown circumstances with Mabel being among them. Shortly afterwards Nick will put Mabel and the rest of Kelsey's family to rest.