What a wonderfull story told by first Helen, then Johnny Cash, who married June Carter and became the son-in-law of Mother Maybelle. The marriage may well have saved the man from your typical showman’s disasters involving drugs and sex and rock & roll.
And another nice thing to note is the aging voice of Maybelle. She was never a singer with soaring voice but with age she becomes even more careful not to stretch it too far. She stays within her modes range and plays an important harmony role. And she does take the lead in going through the song, and she is always very sure of when to start and when to end a particular part. That is often seen in those who are also strong guitar players, I believe because they have two perspectives on song, the chord progression and the melody.
See more about the Carter Family.