black sheep (of the family)
= the worst member of a family or group.

“Brenda is the black sheep of the family. She always gets in trouble with the police.”

A: “My brother has been kicked out of school 3 times in 4 years!”
B: “What do you expect from the black sheep of the family?”

*It’s common to just say (the) black sheep to mean the worst person in a family/group. In the example above you could also say:

A: “My brother has been kicked out of school 3 times in 4 years!”
B: “What do you expect from the black sheep?”

a wolf in sheep’s clothing
= a dangerous person pretending to be harmless.

“My new coworker seemed to be a very friendly guy, but we later found out that he was a wolf in sheep’s clothing. He lied to us and was caught stealing from the company.”