The 30-year-old actress attended a dinner for her sibling Saturday following news Haylie, 33, is expecting another baby with fiancé Matt Rosenberg.
"Was so happy to be home for the weekend celebrating my sister and this new little gift our family is getting! We are so excited," Hilary captioned a photo Monday on Instagram.
"Love you so much @haylieduff you are a beautiful mama and what an extra special night with great friends!" she added.
Haylie thanked Hilary and her friends and family in a post on her own account Sunday.
"Thank you to everyone that came together yesterday evening to make me feel so loved & special. This little girl has so many wonderful 'aunties'.. she doesn't even know how lucky she is yet..." she wrote.
Haylie already shares 2-year-old daughter Ryan with Rosenberg, whom she got engaged to in 2014. She confirmed in January that she's expecting again, and announced the next month that she's having another girl.
"Ryan is thrilled! She really wanted a baby sister," a source told People at the time. "The whole family is so excited for the new baby to arrive."
Hilary herself shares 6-year-old son Luca with ex-husband Mike Comrie.