"(Bindi's) massive now," Robert blurted out the minute his mum had finished her story.
The reaction was instantaneous. The 17-year-old copped a fist in his arm from his mum and a scandalised "Robert!" from recently-married host Samantha Armytage.
And so the backtracking began.
"I mean... in a good way," a defensive Rob started.
"Spoken like a true little brother," Sam said in between laughs.
"She's not here now," Robert said as his face grew an increasingly tomato shade of red.
Kochie chimed in at this point, jokingly asking "she's going to hospital, not the clinic?"
"She's not going to our wildlife hospital, no," a laughing Robert answered the host. "We do have incredible facilities here though.... she could if she wanted to."
Terri jumped to her son's defence at this point, saying, "there's a little spot next to the injured kangaroos and koalas".
But it seemed Robert was merely digging himself a deeper hole as Samantha Armytage claimed, "Robert, you are in so much trouble when you run into your sister after this. She is beautiful".
The wildlife warrior was quick to agree, saying, "I'm in so much trouble. Don't tell her about this please."
Somehow, we think the word will get out.
However, fans of the animal lovers were quick to point out that Robert's loose lips may have been an inherited trait.
"Something his dad would have said," wrote a fan.
Hopefully Bindi will forgive her brother by the time her baby is born next month so that Rob can get in some much-cherished uncle time with the newest wildlife warrior.