The siblings, who performed in the WWE as the Bella Twins, said on Instagram that Laurinaitis will have surgery Tuesday to remove a mass from her brain stem "that has been paralyzing her face."
"Our Mama Bella, our Gigi, our Gato is about to head into brain surgery. I ask for so many prayers, so much light and love sent her way," Nikki wrote.
"I have faith that the amazing doctors will do an incredible job and our Gigi will be on the road to recovery starting late today," she added.
Nikki is pregnant with her first child with her fiancé, Artem Chigvintsev, while Brie is expecting her second child with her husband, Daniel Bryan. Nikki voiced her hope that her mom recovers soon and is able to spend time with her future grandchildren.
"She've loved by so many of us and just pray that her road to recovery is exactly what we are hoping for so we can put two new grand babies in her arms soon," Nikki said.
Brie, who has a 3-year-old daughter, Birdie, with Bryan, shared a photo of her mom and stepdad, John Laurinaitis, with Birdie.
"Today my mom goes in for brain surgery. They realized she doesn't have Bells Pasley but a mass on her brain stem..... couldn't sleep because I was praying all night. She's a strong one and I know all her angels will be with her," Brie wrote.
"#BellaArmy send Mama Bella all the prayers and love today," she said. "Love you Mom @kathylaurinaitis."
WWE stars Sasha Banks and Billie Kay, retired wrestlers Torrie Wilson and Michelle McCool-Calaway, TV personality Maria Shriver and actress Jamie Lynn Sigler were among those to voice their support for Laurinaitis in the comments.
"Sending all my love and good thoughts for your mom @kathylaurinaitis you got this," Banks wrote.
"Sending so much love and prayers to her and all of you," Kay added.
"Sending much love & prayers. We love you Mom!" Wilson said.
Nikki and Brie paid tribute to their mom and other women in their life on Mother's Day in May.
"I'm blessed to be surrounded by so many amazing mamas that have paved such a beautiful way for me to follow," Nikki wrote.
"Happy Mother's Day to my mama, my grandmother, godmother, sister and to all the mother's out there!!! Sending so much love your way," Brie said.
Nikki and Brie announced in January that they are both pregnant and due to give birth within two weeks of each other. Nikki said during the Total Bellas Season 5 finale this month that she is expecting a baby boy.