Original: 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.
Simplified: 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.