When we do harmony singing workshops, we always mention both the Everly Brothers and The Blue Sky Boys as examples of fine harmony singing. In both cases their intonation and phrasing are impeccable, even later in life.

By the way, it is a distinct advantage to, in fact, be brothers because their dialect and accents are identical and this is very helpful in singing close harmony. Don't believe it? Imagine two guys singing harmony with one of them being brought up in Brooklyn, NY and the other in Montgomery, Alabama!