The Kossoy Sisters- Irene and Ellen, whose absolutely magical version of I'll Fly Away can be heard in the film O Brother, Where Art Thou?.
However on the soundtrack CD it is performed by Alison Krauss and Gillian Welch . Permission from the recording company (I think) was withheld thus the re-recording of the song. The sisters recorded their first album, Bowling Green, in 1956 with Erik Darling.

The Kossoy Sisters