“You’re always there for me and I know you will be the best uncle when our baby girl is born. Love you,” she added.
Enthusiastic fans were quick to comment on the post – although a few were seemingly left a little bit emotional by the outpouring of love between the siblings.
“I would like to make it known I am near tears because of how beautiful this sister and brother friendship is,” one gushing fan wrote.
“It’s lovely to see such a friendship between brother and sister. Such a lovely family,” another person stated.
Meanwhile, a third person added: “I love how you two have such a deep connection to each other!”
Robert, 16, has always shared an incredibly close bond with his sister, and in April, he fought back tears while speaking about her on an episode of Crikey! It’s the Irwins.
Ahead of Bindi’s wedding to Chandler Powell, the 16-year-old photographer became visibly overcome with emotion and lost for words.
“I think for all of us, it's just emotional, excited and it's just such an incredibly special day. I'm overjoyed for Bindi and Chandler to be starting their married life today,” Robert began.
Struggling to fight back “happy tears”, Rob tried to describe his feelings for his sister, before getting choked up and walking off camera.
“I've got the most incredible sister in the world and I absolutely just love her so much and I'm so happy for her today," he said.