Complex sentence: Zach takes this opportunity to propose marriage to his son's lover--he propositions Kendall with the deal that they both can get back at Ethan for his betrayal, by marrying each other.
Split into simpler sentences: Zach takes this opportunity to propose marriage to his son's lover. He propositions Kendall with the deal that they both can get back at Ethan for his betrayal, by marrying each other.