The back-sheep of the family, refers to the family member who never quite fit in, who maybe everyone loved, but who no one wanted to really talk about.
My paternal grandfather had a couple of brothers who were the black-sheep of the family. The bartender whose two wives showed up to his funeral (his nickname was Dutch,) and then there was the gambler and pool hustler who lived very well and seldom had a real job (Bud.) My father's generation was tiny, just him and his sister, both of them fit well; my mother was an only child. My maternal grandmother was an odd duck, there were many members of her family who didn't talk to or about other members of the family.
My generation is a bit more complicated. I am the youngest of four. I was the first to finish college, the first to get a divorce, the only one to come out of the closet. That probably qualifies me as a black-sheep - I treasure the role of the underdog. One of my brothers has spent most of his adult life isolating himself from the family. My middle brother is just himself, he has made his way in the world largely as a solo act. Both of my brothers likely fall into the black-sheep category. My sister is the only one who took the traditional route of marriage and children.
Who is the underdog in your family?