The pro beach volleyball player shared a photo of her little girl in her Instagram stories, writing, "Today would have been your 3rd birthday. We miss you so much."
Emmy was just 19 months old when she died after drowning in a neighbor's pool in 2018.
Miller and her husband, former Olympic skier Bode Miller, announced in August that they are expecting identical twin boys. The couple welcomed a son, Easton, last October, just four months after Emmy's tragic passing.
Morgan opened up to her Instagram followers about how "(l)osing a child while pregnant was the most confusing experience of my life."
Once Easton arrived, however, he helped his mom feel joy again.
"Easton provided us an even closer bond to his sister," she wrote in August. "The moment I heard his cry, something sparked back alive in my soul. Hope. Love. I’m not sure. But in that moment, I knew I was Mom and my kids deserved the world from me. Everything was going to be okay. My joy and grief could coexist."
The couple also have an older son, Nash, 4. Bode has a daughter, Neesyn, 11, and son, Sam, 6, from previous relationships.
While their family is growing, Morgan said they "carry Emmy with us every day."
"That part doesn't ever get easier, but we just imagine what it would be like to have her there, and we share our tears and we share our memories and we revisit stories," she told TODAY in August.
"Keeping her a part of our family has really made this journey a little bit easier for our family."