Complex sentence: Rosette works at a casino whose owner, Jake Stillman (Bruce Cabot), demands that she marry Joe, whom Jake plans to kill once he is married to Rosette, so that he can marry her and gain the fortune in gold.
Split into simpler sentences: Rosette works at a casino, who's owner, Jake Stillman (Bruce Cabot), demands that she marry Joe. His plan is to kill Joe once he is married to Rosette, then marry her and gain the fortune in gold.