On Sunday, Robert, 17, shared a sweet photograph of himself and his older sister, 22, to mark National Siblings Day. The image features the Irwin siblings posing next to each other as Bindi holds onto her newborn daughter, Grace Warrior, and Robert rests his hand gently on the infant.
"Happy #NationalSiblingsDay!" Robert wrote alongside the Instagram post. "I've got the absolute best sister - Grace definitely has an amazing mum 💙."
Their mother Terri Irwin also shared throwback photos of the siblings on Twitter.
In the comments section, an array of fans of the Crikey! It's the Irwins stars left sweet messages for the brother-sister duo. "You guys are both so lucky to have each other!" one fan wrote as another said, "best siblings ever ❤️."
Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories.
One other fan added, "❤️❤️❤️ the future of Australia zoo right there."
Meanwhile, another fan mentioned the pair's father, Steve Irwin, and wrote, "Keep up the good work to make his legacy alive ❤️." Steve — who was commonly known as The Crocodile Hunter — died during a diving expedition off the coast of Australia in 2006.
RELATED VIDEO: Pregnant Bindi Irwin Says Late Father Steve Would Have Been a 'Perfect' Grandpa
Last month, after Bindi welcomed her daughter into the world with her husband Chandler Powell, Robert shared a sweet photo on Instagram showing himself holding his newborn niece.
"Let the uncle adventures begin!" he wrote in the caption. "Love you so much, Grace ❤️ This little one picked the two best parents in the entire world. The most incredible, caring and strong Mum... and the funnest, coolest and kindest Dad. Love you three so much - I can't wait for this exciting journey ahead!"
In the comments section, new mom Bindi wrote, "Thank you for being the greatest brother. Grace loves you so much! We all do."
Additionally, Powell, 24, added, "We all love you Funcle Robert!" hinting at Robert's nickname as the "fun uncle."