The US evangelist tweeted to say his older sibling - the second daughter of the late Rev Billy Graham - was still undergoing chemotherapy.
He said: "It's been tough, but I can't tell you how much the prayers have meant to her and to our entire family. Thank you for praying - please continue."
The post, which Anne Graham Lotz shared on her own Facebook page, attracted hundreds of supportive comments.
Nita Heigert replied, "Praying for total and complete healing with minimal complications from the chemo", while Richard McClain said, "[I] pray for you every day."
My sister Anne @AGLotz is undergoing chemotherapy. It's been tough, but I can't tell you how much the prayers have meant to her and to our entire family. Thank you for praying—please continue. pic.twitter.com/zotntAWtIC— Franklin Graham (@Franklin_Graham) January 9, 2019
Another response, published by Diane Bridgeman Jendresen, said: "You are such a blessing to me. You are in my heart and in my prayers."
Lotz (pictured below at the funeral of her father in 2018) said before Christmas that her cancer - diagnosed in August 2018 - had "required immediate, prompt, drastic surgery" and is "potentially fatal".
She wrote on her website: "It struck swiftly and unexpectedly.
"It required immediate, prompt, drastic surgery and follow-up treatments that include chemotherapy and radiation.
"Through our prayers and God's answers, I believe I will be victorious over it."
Lotz's role as a preacher has taken her to venues around the world.
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