Bindi Irwin missed the annual Steve Irwin Gala after undergoing emergency surgery.
The 26-year-old conservationist had been due to attend the annual fundraiser - held in honour of her late dad, crocodile hunter Steve Irwin - on Saturday but she had been taken to hospital after her appendix ruptured.
Her mum Terri Irwin, 60, also missed the event to be by her side.
"She's going to be OK, but surgery - out of all the things we were ready for, that was not one of them," Bindi's brother, Robert Irwin, told People magazine.
"She's just come out the other side of endometriosis and now the appendix goes. Health is so important - it really is.
"Bindi has become an incredible advocate for women's health, particularly, and I think it's really important to prioritise getting help."
Irwin, 21, revealed his sister had been determined to ignore the pain she was in and attend the gala, but doctors intervened.
"She came to Las Vegas and was ready to come to the gala, put on a brave face in a lot of discomfort and a lot of pain and said, 'Nope, I'm just going to tough it out, I'm going to go for it'," he said.
"But the surgeon said, 'No, your appendix is going. That thing's gotta come out.' Health has to come first.
"She's very sad. She's devastated that she and mum can't be here, but I know she'll make a speedy recovery."
Just a day before, Bindi Irwin had shared her excitement about being in Las Vegas for the gala.
She wrote on Instagram: "Khaki by day - Bellagio chic by night! Here in #LasVegas for the #SteveIrwinGala raising funds and awareness for Wildlife Warriors.
"Supporting conservation and remembering Dad's extraordinary legacy. Enormous gratitude to everyone for your support as we continue to do all that we can to protect wildlife and wild places for the future generations."
Bindi - who has a three-year-old daughter, Grace, with husband Chandler Powell - previously underwent explorative surgery after battling through more than a decade of pain and extreme fatigue.
Doctors finally concluded she was suffering from endometriosis, which causes tissue similar to the lining of the uterus to grow in other places.
She later explained the surgery had uncovered 37 lesions and a cyst and she felt as if she had finally been given "validation".
In a post on social media, she wrote: "For 10yrs I've struggled with insurmountable fatigue, pain and nausea. Trying to remain a positive person and hide the pain has been a very long road.
"These last 10yrs have included many tests, doctors visits, scans, etc. A doctor told me it was simply something you deal with as a woman and I gave up entirely, trying to function through the pain ...
"I decided to undergo surgery for endometriosis ... Every part of my life was getting torn apart because of the pain. To cut a long story short, they found 37 lesions, some very deep and difficult to remove, a chocolate cyst.
"(The doctor's) first words to me when I was in recovery were, 'How did you live with this much pain?; Validation for years of pain is indescribable."