Complex sentence: Sienna then tells Nancy to run away with Oscar and Charlie Dean (Charlie Behan), but when Nancy leaves, Sienna harms herself and fills her flat with gas to make it appear that Nancy has tried to kill her.
Split into simpler sentences: Sienna then tells Nancy to run away with Oscar and Charlie Dean (Charlie Behan). But when Nancy leaves, Sienna harms herself and fills her flat with gas to make it appear that Nancy has tried to kill her.