Complex sentence: Bertha surmised that Selden had come to Bellomont to see Lily not Bertha, and she retaliated immediately by making sure Percy found out about Lily's gambling, smoking, and borrowing money from men to pay off her gambling debts.
Split into simpler sentences: Bertha surmised that Selden had come to Bellomont to see Lily not Bertha. She retaliated immediately by making sure Percy found out about Lily's gambling, smoking, and borrowing money from men to pay off her gambling debts.