Close bond! In honor of Robert Irwin‘s 16th birthday, sister Bindi Irwin decided to reveal just how she feels about her little brother.

“Happy 16th Birthday to the cutest ducky I’ve ever known!” Bindi, 21, wrote via Instagram alongside a throwback pic of her younger sibling. “Robert, one of the best parts of my life is getting to be your sister. Thank you for being such a wonderful light in my life. You’re amazing and I love you!” Take a look at the sweet pic below!

Fans were head over heels for this adorable post, as they rushed to the comments section to respond. “Happy birthday Robert!! Honestly one of the coolest people! Have an awesome day. I wish I could make it to the zoo but I am sending love,” one person said. Another added, “Aww! Your family is such a beautiful light in this world! Thank you for your positivity.”

It is quite clear that Steve Irwin‘s daughter is very close to Robert. In fact, the conservationist even wants him to walk her down the aisle at her wedding in place of their late father. “That’s something that I really want to do. I’m always so grateful that Robert is there, and he has been such a big part of my life and he’s always stepped up and been the one to give me a hug when I need it and encourage me forward in life,” Bindi explained to ET. “So he will definitely be the one to walk me down the aisle when the time comes, and I think that will make it really special.”

Robert was also all about it. “I was just so so stoked. I was so excited and really honored for her to ask me to walk her down the aisle,” he told People at the time.

The TV personality also wrote via Instagram, “Dad, you would be so incredibly proud of Robert. … I know you’d be beaming with pride when the time comes that he walks me down the aisle. I wish you could be here for these moments, but I know that your spirit lives on in us.”

It is so incredible to see this duo’s connection.