The Million Pound Drop presenter broke the news on Twitter earlier this evening (August 1).
She wrote: "My beautiful brave sister Caroline (@madfroggy) lost her fight against cancer today. My heart hurts so much."
She also posted a picture of Caroline smiling from her hospital bed, with the caption: "My amazing sister."
McCall was inundated with thoughtful messages from her fans in the wake of the news.
"Thank you for your kind messages xx," she wrote, later repeating the post and adding: "Night night xx."
Phillip Schofield, Dame Kelly Holmes, Emma Bunton and Brian Dowling were among some of the famous faces to offer their condolences on the microblogging site.
Caroline is McCall's mother's daughter from her first marriage. She reportedly lived with McCall and her family in Surrey for a time prior to her passing.