The 39-year-old Gaines, who is pregnant herself, posted a photo of her sister in the hospital. Gaines is seen holding her newest niece.
"My sister had her beautiful baby girl!" Gaines wrote alongside the hashtags, "mother of six" and "but she looks 16."
McCall and her husband, David, welcomed their fifth child in February 2015. Gaines posted a photo celebrating that birth on Instagram as well. McCall also appeared on Tuesday's episode of Fixer Upper, as Gaines and husband Chip worked on McCall's Waco, Texas home.
Chip and Joanna Gaines surprised fans on Tuesday by announcing that they are expecting their fifth child. "Gaines party of 7.. (If you're still confused.. WE ARE PREGNANT)," Chip wrote in an adorable photo that showed him with a 'baby bump.'
Gaines also posted a video of the ultrasound, showing the baby's heartbeat. "Chip swears he can already tell it's a boy. Look at the little heartbeat," Gaines wrote.
Chip and Joanna Gaines are already parents to Drake, 12, Ella 11, Duke 9, and Emily Kay, 7.
While many of their fans were happy about the news, some wondered if they are having a baby to try to save their marriage.
"Wish I could be happy for them...but no one needs five children. There are too many people in this world as it is. The world can't support the reproductive rate at this pace," one fan wrote.
New episodes of Fixer Upper air on HGTV at 9 p.m. ET on Tuesdays. The current season will be its last.
Photo credit: Instagram/Joanna Gaines