Maggie Grace , who portrays Shannon Rutherford , Boone 's stepsister , was asked to return at the beginning of the season , but Grace was unable due to her schedule ; she appeared later in the season , instead . Maggie Grace , who portrays Shannon Rutherford , Boone 's stepsister , was asked to return at the beginning of the season , but Grace was unable due to her schedule ; though she eventually did make an appearance in one episode .