Heidi, 28, confirmed the news on Twitter over the holidays, tweeting a link to a picture of Holly's baby bump. "Couldn't be happier for my sister @hollymontag Christmas miracle!!!!!" she wrote. "Love you!!!"
The photo, taken before the New Year, shows the mom-to-be, 31, cradling her pregnant belly while posing with Wilson beside a Christmas tree. According to MTV News, she's about five months along and is due to give birth in May 2015.
This will be the first child for Holly and her husband, whom she married in their hometown of Crested Butte, Colo., on Sept. 14.
"It could not have been more intimate and elegant," the bride previously told Us Weekly of her wedding. "Being surrounded by the mountains as the trees are just beginning to turn for fall was absolutely dreamy. It could not have been more magical or perfect for Richie and me."
Added Heidi: "I couldn't be happier for my sister finding the love of her life and soul mate. I pray and wish them all the blessings and happiness in the world. She deserves it!"