Reality star Sadie Robertson celebrated the first annual World Adoption Day on Sunday by sharing a cute photo with her mother, and shouting out her adopted siblings.

Robertson's parents, Willie and Korie, have five children: Rebecca, 24, John Luke, 19, Sadie, 17, Will, 12, and Bella, 11. Rebecca was an exchange student that the Robertsons fostered and later adopted, and "Lil Will" was adopted by the family when he was five weeks old.

Sadie expressed love for her non-biological siblings in a Twitter post.

"Happy #worldadoptionday we love our lil will and Rebecca!" she wrote. "Couldn't imagine life without my lovely brother & sis!"

In the accompanying picture, Sadie and Korie have smiley faces drawn on the palm of their hands – a symbol of World Adoption Day.

Korie, 41, has been a strong advocate for adoption, and was honoured, along with her husband, by the Congressional Coalition on Adoption Institute last year.

"From the moment we found out about [Lil Will], he was ours," she said at the Angels in Adoption Gala. "I swear, I had labor pains the night before I went and picked him up at the foster home he was living at. He's our child, just like our biological children."

Willie also described how much he loves his namesake.

"I've done a lot of cool things in the past couple of years, I've been to Super Bowls, I've met Presidents – but the best decision we ever made – that made the biggest impact on our life was the adoption of our son Will," he said.

The couple also said that they would consider adopting another child.

"There are plenty of kids who need homes," Willie told In Touch Weekly last year. "That's why we're all here. We have adopted children and we have natural children ... and there's no difference between them."