Atticus and his family are not typical at all in Maycomb. This does not have to do so much with their long lineage. Maycomb is a place where people have settled down for generations. What makes Atticus and the Finch family atypical is that they are broadminded, courageous, and sacrificial.
Frist, Maycomb is a place where people are racist. In fact, the racism runs so deep that they cannot even see it. It is such a part of their worldview, that even when all evidence points in favor of the innocence of Tom Robinson, the people cannot see it. Hate blinds, and there is plenty of hate in Maycomb. Atticus and his family are exceptions. They are broadminded; so, they see the racism and try to do something about it.
Second, the Finch family is also courageous. For example, Atticus knows that he will not win the case, but he will commit and try his best. In one of my favorite parts of the book, Atticus offers his prediction of the case to Scout:
“Atticus, are we going to win it?"
“Simply because we were licked a hundred years before we started is no reason for us not to try to win,” Atticus said.
Third, the Finch family is willing to suffer to do the right thing. Consider the mob scene. These men were going to kill Atticus, but Atticus still stood his ground. This kind of courage is rare in Maycomb. We also see this is Jem, who defended his sister when Bob Ewell attacked.
In conclusion, the Finches are a minority in Maycomb.