If you have a cousin who seems to be the one troublemaker in your family, you can call her the black sheep. In other words, she doesn't seem to fit in with the rest of your family members.
The black sheep is the odd one out, whether he's a disgrace to the family or just doesn't seem to belong. The origin of the phrase comes from the rare presence in a flock of white sheep of a sheep with black fleece. Rather than being a delightful surprise, these black sheep were a disappointment, since their wool couldn't be dyed.