Sister of Las Vegas shooting victim describes her last moments: 'She was literally dying in my arms'

— -- Andrea Castilla had been out celebrating her 28th birthday at the Route 91 Harvest Music Festival in Las Vegas with her sister and her boyfriend, Derek Miller, who had been waiting for the right moment to propose.

“I told him, she would have said ‘yes’ immediately,” her sister, Athena Castilla, told ABC News’ Juju Chang for “Nightline.

The country music fans were having the time of their lives at the festival, dancing and singing to the music near the main stage on Sunday night, when suddenly, terror struck.

A gunman identified by police as 64-year-old Mesquite, Nevada, resident Stephen Paddock, opened fire and sent roughly 22,000 terrified concertgoers running for their lives. The attack killed 59 people, including the gunman, and injured nearly 500. Paddock was found dead on the 32nd floor of the Mandalay Bay Resort and Casino when authorities entered his room. The hotel was across the street from the music festival.“My sister said, ‘duck,’ so we duck and the next thing we know she was already hit,” Athena said. “It just happened so fast.”

Andrea was shot in the left side of her head and fell to the ground. Her sister immediately tried to shield her from the gunfire and tried to comfort her.

“She was struggling to breathe and I just held her head,” Athena said. “We were trying to find the wound… I’ve never seen so much blood in my entire life.”

As her sister lay on the ground, Athena said people started running all around them. Eventually, she and others were able to grab a barricade gate and use it as a makeshift stretcher to carry Andrea out of the venue. The entire time, Athena said, she was still trying to talk to her sister, and told her, “You’re our angel, you’re going to be fine.”

“I was like, ‘Don’t leave me,’” Athena said.

They finally got her to a pick-up truck that was taking victims to the hospital.

“We couldn’t give up on her,” Athena said. “She deserved a chance to get help…she was literally dying in my arms.”

But after Andrea arrived at the hospital, the family said they were mistakenly told that she was still alive and they held out hope for several hours until they learned the truth. Andrea died that night.

“I was trying to stay positive that she was going to make it, but I could see all the blood,” Athena said. “It was all over me.”

The family had already lost their mother to cancer, and Athena said her sister had wanted to use her talents as a makeup artist to help cancer patients feel beautiful. She and their brothers said they are still in shock by what happened, and her boyfriend Derek is heartbroken.

“[Andrea] was a beautiful soul,” said her brother, Adam Castilla. “She shouldn’t have died that night.”

ABC News' Jasmine Brown and Ashley Riegle contributed to this report