Which country singer was a frequent duet partner and also his mother-in-law?

The country singer who was Johnny Cash's mother-in-law and a frequent musical collaborator was Maybelle Carter. Cash became part of the Carter family through his marriage to June Carter, Maybelle's daughter, in 1968. Maybelle subsequently toured frequently with Cash and appeared with the Carter family on his television work.

Maybelle was a founding member of the Carter Family, one of country music's earliest commercially successful groups. She was renowned for her guitar technique, often called the Carter Scratch, in which her thumb carried melody lines while her fingers supplied rhythm. That approach helped establish the guitar as a lead instrument in country music.

Maybelle's connection with Cash was more than family. She played and sang on recordings associated with him, including the 1973 single “Pick the Wildwood Flower,” and appeared on his television variety show. The distractors are close family names: Anita and Helen were Maybelle's daughters, while Sara Carter was her cousin and fellow original Carter Family member.