He started dating his wife because she was his sister's best friend and years later after they got married his sister told her she and her brother used to have sex before she started dating him. She didn't got hungry or jealous she even liked the idea of sharing his husband with her best friend again. And it finally append, when his sister lost her job and with it her home. She stayed at her brother's place until she gets on her feet again, but already passed one year and no one wants things to change. This is how is living with his wife and his sister.