Definition of ugly sister in English:

ugly sister

noun

figurative
  • A person or thing considered unattractive, inferior, or unpleasant compared to others of the same type or group; an unpleasant or undesirable counterpart.

Origin

Late 19th century; earliest use found in Frederick Boyle (b. 1841). With allusion to Cinderella's ugly and unpleasant stepsisters in the fairy tale Cinderella.