Hooked on the Wrong Brother

On her eighteenth birthday, Lily mistakes Ethan—her boyfriend Brad's stepbrother—for Brad and sleeps with him. The next day she catches Brad cheating with Chloe, and Ethan uses her secret to force her into being his tutor. Chloe keeps scheming against Lily, but Ethan exposes every plot. After Lily dumps Brad and falls for Ethan, Chloe spreads a rumor that Lily is just a stand-in for Ethan's first love, and Lily blocks him.