Complex sentence: Slim gives Curley an ultimatum; not to tell anyone what exactly happened and if he does tell his father in retribution to get Lennie and George fired, he will tell everyone what happened.
Split into simpler sentences: Slim gives Curley an ultimatum: not to tell anyone what exactly happened. If Curley does tell his father in retribution to get Lennie and George fired, Slim will tell everyone what happened.