Someone who differs from the rest of their family by being relatively normal. The other members may be wild partiers or mentally unstable or anything else with a generally negative connotation while the Albino Black Sheep is tame/sane.
Bert: Yo, I had a foursome last night with the triplets Becca, Betty, and Bernice. Should I go for their sister Brittany tonight?
Bob: Nah dude, she's an Albino Black Sheep, she's not as easy as her sisters.