"Happy birthday @BindiIrwin. You've always been the best sister, and now it's so cool to see you as such a wonderful mum to little Grace! I'm so proud of you! (Ps. You and Chandler are raising an absolute fashion icon," Robert captioned the picture of him smiling with Bindi, her husband Chander Powell and their infant daughter Grace, who is wearing heart-shaped sunglasses and a hat.
"Grace's face is a MOOD!! Thank you so much. You're the best brother in the world," Bindi responded to Robert's message.
Bindi Irwin married Powell in March 2020 and the couple welcomed their first child a year later.
The family is known for their wildlife docu-series, The Crocodile Hunter and Crikey! It's the Irwins. Bindi Irwin also was the winner of Season 21 of Dancing with the Stars in 2015.