The younger brother's current problem while studying for entrance exams is that he sees his older sister in a sexual way and is unable to study. When her sister pointed this out to her, she took the plunge and begged her for sex...and of course she refused. However, her younger brother doesn't give up and suggests, "If your pussy doesn't work, let me lick your anus!" The older sister reluctantly accepts, but she gradually becomes addicted to having her anus licked... her body can't stand it without her brother's anus licked!
