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Dolly Parton ’s sister has fans on edge -- revealing online the music legend ain’t doing too good -- and she's begging for prayers.

Not long after Dolly axed 6 shows from her Vegas residency due to health reasons, her sister Freida hit Facebook Tuesday ... saying the 79-year-old legend still isn’t at her best, and pleading with fans worldwide to be "prayer warriors" for the country legend.

Freida -- who said she’d been up all night praying -- reassured fans that Dolly’s strong and deeply loved ... and with the world’s prayers lifting her up, she believes her sister’s gonna be just fine. Getty

She wrapped the post with, "Godspeed, my sissy Dolly. We all love you!" -- alongside a powerful shot of Dolly herself in prayer.

Of course, fans didn’t need to be told twice ... Fr

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