Complex sentence: The "General Slocum" disaster was the New York area's worst disaster in terms of loss of life until the September 11,2001 attacks, and remains the worst maritime disaster in the city's history.
Split into simpler sentences: The "General Slocum" disaster was the New York area's worst disaster in terms of loss of life until the September 11,2001 attacks. It is the worst maritime disaster in the city's history.