Thank you for listing all the versions, Richie,
from glancing over all the Hicks/Harmon/Presnell versions each one of them is a bit different in details, if they all came from the same family tradition they might have been influenced by other traditional versions (and forgetfulness) to various degrees.

I'll give the links to the texts here for others to compare them easily:
Nora Hicks: http://www.bluegrassmessengers.com/the-two-sisters--hicks-nc-pre1939-walkerabram
Hattie Presnell: http://www.bluegrassmessengers.com/two-sisters--hattie-presnell-nc-1966-burton.a
Lee Monroe Presnell: http://www.bluegrassmessengers.com/two-sisters-that-loved-one-man--presnell-nc-1

Nora Hicks', Hattie Presnell's and Lee Monroe Presnell's versions are closely related, but Hattie's and Lee Monroe's are missing the ending completely.
Nora Hicks' version is the most complete US version I've seen so far, rivaled only by Aunt Becky Gordon's "Edinboro" version (See Brown C).
It's very unfortunate that no sound recording of Nora's version was made, I know that she was recorded by Frank C. Brown and William Amos Abrams, but they apparently missed this song.

Jane Gentry: http://www.bluegrassmessengers.com/the-two-sisters--jane-gentry-nc-1916--sharp-a
Maud Long: http://www.bluegrassmessengers.com/the-two-sisters--maud-long-nc-1955-rec-moser-

Jane Gentry's and Maud Long's are still similar, but Maud's has picked up the opening stanza of another standard "Bow Down" version.

One stanza:
O, as they walked down to the water's brim
The oldest pushed the youngest in.

Is very different in Jane Gentry's:
She picked her up all in her strong arms
And threwed her sister into the sea.

Maud Long's version has also replaced the harp ending with the standard "miller robs her and gets hanged" ending from the "Bow Down" versions.
I think you are right about the singer trying to get a more complete text for the collector, Steve, perhaps Maud Long has heard the "Bow Down" version of the ballad somewhere and filled out her own version with that.

Mrs. Samuel Harmon: http://www.bluegrassmessengers.com/the-two-sisters--harmon-tn-1930-henry-c.aspx

Mrs. Samuel Harmon's version is a bit like a missing link.
It is missing much of the story, but it has the Presnell opening stanza:
1. Was two sisters loved one man,
Jelly flower jan;
The rose marie;
The jury hangs o'er
The rose marie.
2. He loved the youngest a little the best,
Jelly flower jan etc.

The refrain at the end of each stanza is like Jane Gentry's version.

It has the harp stanzas, but here it's "she", apparently the bad sister herself that makes the harp.

The harp then plays:
8. The first string says, "Yonder sets my sister on a rock
Tying of a true-love's knot,"
9. The next string says, "She pushed me in the deep so far."

Which recalls an earlier stanza from Jane Gentry's version:
7. O the farmer's wife was sitting on a rock,
Tying and a-sewing of a black silk knot.

I guess that Mrs. Samuel Harmon's ending was once part of Jane Gentry's version too, but eventually forgotten.

Those are my observations for now.

-Kevin W.