I personally think that their beautiful harmony was of the type that can only be achieved by siblings. Not only was their harmony excellent but also their sense of timing was perfect.

Sundance, with your appreciation of tight harmony, you may enjoy the extremely tight 3-part female harmony of The Wailin' Jennys (qv) a new discovery for me out of Canada. My speech therapist, Christine, recently introduced me to their music although they have been active for several years.

Let me know what you think of them. They have many beautiful songs on YT.