In his post, Sadler wrote that his wife had been kept on life support because she was an organ donor.
"Even at the end she is still giving. She was so loved that I know thousands of you will be grieving with us. There are only a few hours left for me to hold her hand and kiss her forehead," Sadler wrote.
He added that McKibbin was a longtime fans of Stevie Nicks and that Fleetwood Mac's 1975 song "Landslide" would be played for her before doctors performed her final surgery.
"If you are able, you can pause at 3:00 wherever you are and listen to it with her," Sadler wrote. "She will know that you're sharing her farewell. She loved so many of you and I know you loved her too."
McKibbin is best known for placing third on the debut season of American Idol in 2002, finishing behind runner-up Justin Guarini and champion Kelly Clarkson.
During her time on the popular music competition show, McKibbin blew viewers away with covers of Nicks, Alanis Morissette, Melissa Etheridge, Janis Joplin and others.
Her 15-year-old son, Tristan Langley, auditioned for Idol in 2014, but was cut during the Hollywood rounds.
McKibbin's struggle with alcohol and substance abuse was later chronicled on Celebrity Rehab With Dr. Drew in 2008, and a subsequent spin-off of the show, Sober House.
Fellow Idol contestant Guarini remembered McKibbin as a "fiery, funny lady who could sing the hell out of a rock song" in a tribute post on Instagram.
"I’ll never forget the day that her idol, Stevie Nicks, sent her flowers with a card that said 'You are the Gypsy that I was,' Guarini wrote Saturday. "Nikki was on cloud nine and the joy and excitement that radiated from her was infectious. After the kind of emotional beating she took at the hands of the masses, she deserved to feel good about herself, even for a little while."
He added, "Rest well, 'Gypsy'...and thank you for the laughter, merciless teasing, strength, vulnerability, love and friendship you showed me during our time together in the spotlight."