Holly, 31, is expecting her first child with husband Richie Wilson, a situation confirmed by Heidi on Twitter on December 23.
At that point, Holly was about five months into her pregnancy, as reported by MTV News.This has been a busy autumn for Holly and Richie, who married in Crested Butte, Colorado on September 14.
The pregnancy wasn't new when Heidi spilled the news to the world by tweeting a link that showed Holly and her baby bump.
'Couldn't be happier for my sister @hollymontag Christmas miracle!!!!! Love you!!!' Heidi wrote in the message.
In the photo, Holly posed next to a Christmas tree with her husband - a fitted white sweater clearly revealed her pregnancy.
A pleased Holly quickly wrote back to her little sister, saying: 'you are the best sister <3 love you so very much!'
While not as famous as her younger sister, 28-year-old Heidi, Holly also gained fame via The Hills between 2007 and 2010.
Holly appeared in 25 episodes of the MTV reality series, featured as part of Heidi's family and for her attempts to start an acting career in Los Angeles.
The elder Montag next appeared on the reality competition, I'm A Celebrity, Get Me Out Of Here! in 2009.
She actually went on that show to replace Heidi and husband Spencer Pratt after the pair quit the show amid controversy.
Holly ended up placing sixth in the competition.
