If you were to type, for example, two sisters into the "Digitrad and Forum Search" box on the main Forum page, you would find a lot of previous discussions of some of the many versions of this song. At least one of them would be this: Lyr Req: I'll be true unto my love
This is a traditional American set of the song, recorded by Clannad amongst others (so that many people now imagine it to be Irish); I suspect that Ms. Parsons learned her set from that Clannad record, so it may be what you are looking for. If you are interested in looking further into the background, there is quite a big list of links to material here and elsewhere in this previous discussion: