Peter Riva, the late actress' grandson said the two were only "great pals" and Valerie Hemingway, the author's daughter-in-law, said "there is absolutely no evidence of there being an actual affair," ABC News reported Thursday.
Riva and Hemingway discussed 30 letters written from 1949-59 donated to the John F. Kennedy Library and Museum in Boston by Dietrich's daughter four years ago but won't be made public until next week.
"I think that it was a flirtation and they were both very large personalities," she said.
"Whenever he was free, she wasn't. When she was free, he wasn't," Riva said.
Hemingway wrote Dietrich in 1950 that "I fall in love with you bad and you're always in love with some jerk" and Dietrich told him, "I want to kiss you forever and a day ... I can't love you more than I do or deeper or longer."
The two were known for frequent romantic liaisons with other partners.
Hemingway was 61 when he committed suicide in 1961. Dietrich died of kidney failure in 1992 at age 90.