Two best-friends, often mistaken for sisters and in some cases twins. They play along when people ask if they are related. Personalities are so similar that people expect them to have been with each other since birth.
boy 1: look at those 2 girls, look like the perfect pair of twins for you and me :P
boy 2: dude, they're not even related
boy 1: they told me they were...lambtwins again :(