The bond between the Irwin children is clearly a strong one and Bindi's followers gushed over the sweet message she shared for her brother.
"You two have such a pure and loving relationship," one fan remarked.
"Happy Birthday Robert! I cried looking at these pictures btw, I don’t know why I just did," revealed another.
"The world is so proud of the people you two have become! It brings my heart so much joy to see you keeping your dad’s legacy alive. Happy Birthday Robert!" a third remarked.
Bindi regularly shares tributes to her brother on social media and has remarked how great an uncle he will be to her daughter, due in 2021.
"You’re always there for me and I know you will be the best uncle when our baby girl is born. Love you."
Robert also shares his love for his big sis regularly, most recently re-posting an image of himself and Bindi with the caption: "Thank you for being the greatest sister ever!!"
For Bindi's 22nd birthday in July, the wildlife photographer shared his own heartfelt post alongside an image of himself as a newborn surrounded by Bindi and his late dad, Steve Irwin.
"Happy birthday @bindisueirwin. Thank you for being my best friend from the moment I entered the world," he wrote at the time.
"I’ll always treasure the story of the first time you met me - it was around the time this photo was taken, just after I was born and you decided that it was your job to name me... and then promptly named me Brian! Dad then confirmed that my name was actually Robert but you still called me Brian 'for short'!
"Can’t wait to celebrate today, I am so proud to be your brother! Love, Brian."