Robert Irwin Holds Newborn Baby Niece Grace in Sweet Photo: 'Let the Uncle Adventures Begin!'

Robert Irwin is officially an uncle!

The 17-year-old shared a sweet photo on Instagram Friday morning showing himself holding his newborn niece Grace Warrior Irwin, whom his sister Bindi gave birth to on Thursday.

For more on Bindi Irwin's baby news, listen below to the episode of PEOPLE Every Day!

"Let the uncle adventures begin!" he writes 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 comment section, new mom Bindi, 22, writes, "Thank you for being the greatest brother. Grace loves you so much! We all do."

Additionally, Powell, 24, adds, "We all love you Funcle Robert!" hinting at Robert's nickname as the "fun uncle."

Over on Twitter, Robert says, "I can't believe I'm an uncle!!! (Or funcle should I say). I love Grace so much already! And she has the two best role models ever - you and Chandler are such amazing parents."

Want to get the biggest stories from PEOPLE every weekday? Subscribe to our new podcast, PEOPLE Every Day, to get the essential celebrity, entertainment and human interest news stories Monday through Friday.

Robert recently told PEOPLE that his sister was due "any day now," adding that she was doing well and "ready to go."

He said at the time, "We are all crossing our fingers. I think it's going to be any day now. She's doing great, she's glowing. She's getting to the point where she needs this baby out of there now. She's a very small person with a very big belly at the moment."

New grandmother Terri Irwin, 56, shared a congratulatory message to her daughter on welcoming her first child.

"Love is not a big enough word," she tweeted. "My heart is so incredibly happy. And I know that Steve would be beyond proud. Grace is the next generation to continue his mission and message of conservation. She chose her parents wisely. Bindi and Chandler are already the best parents ever!"