Mary-Jo Cassidy is the obese sister of both Phil Cassidy and Louise Cassidy-Williams and lives in a home in Little Havana, Vice City. Mary-Jo is envious of her sister Louise and is desperate for attention from anyone. She appears to have a crush on Victor Vance, although this is not elaborated on.
She is, however, very concerned when her sister is kidnapped by both Marty Jay Williams and later by Armando Mendez. During the mission Light My Pyre she says to herself "one day it will be [her]" and is adamant in telling Vic that Armando wanted her as Latin men love women of her type. It is implied that she takes care of Mary-Beth Williams, the daughter of Louise and Marty, after her sister's death.
Her relationship with her brother Phil is not elaborated and she is not mentioned by Phil in Grand Theft Auto: Vice City, Grand Theft Auto: Liberty City Stories or Grand Theft Auto III, probably just because the character was created while in development of GTA:VCS which simply was released after the aforementioned games.