The Jolene hitmaker died in August aged 80 and it later emerged she had been through a "brief battle" with the disease.
But she didn't even tell all her family about her health struggles in order to avoid upsetting them.
Speaking to Gayle King on CBS Mornings, Dolly's sister Stella Parton said: "Well, Dolly did not want anybody to tell that because Dolly could not bear negative things and she would not have wanted her fans worried about it.
"She didn't want a lot of the family worried about it. Some of us knew, the ones that were closer, but not every relative.
"She did not want people sad."
And Stella has too much "respect" for her late sister to share her specific cancer diagnosis.
She explained: "She didn't [explicitly] tell us that we could say it.
"So out of respect for her ... it's not my business to tell her business."
The 9 to 5 hitmaker was "prepared" for her death and the last thing she asked from her loved ones was prayer.
Asked what she misses most about her sister, Stella said: "Her optimism.
"I don't miss her struggling in pain. I know she was at peace.
"She knew where she was going. She wasn't going anywhere bad. She was definitely prepared."
Stella believes Dolly would have been heartened by the outpouring of love that followed her death because it would have cheered her own family, as well as the relatives of her late husband Carl Dean and best friend Judy Ogle.
Stella said: "She'd be really happy and delighted that people have shown her so much love because she knew that that would make us feel better as her family.
"Bottom line, end of the day, her family and Carl's family and probably Judy's family are the people that she would want to say, 'See, everything I did, they cared. They really loved me.' "
Dolly's cancer diagnosis was revealed by her representatives after her death.
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