As Jackie Evancho prepares for Inauguration performance, her transgender sister Juliet Prepares for gender confirmation surgery

— -- Jackie Evancho has been called the “girl with the voice of an angel.”

At just 16 years old, the classical singer has traveled the world. But in a matter of hours, she will step out on the biggest stage of her career to sing the national anthem at President-elect Donald Trump’s inauguration. The move has triggered an onslaught of criticism.

But as Jackie prepares to perform, her 18-year-old transgender sister, Juliet Evancho, is also preparing for a life-changing event -- undergoing gender confirmation surgery.

“Normally, we’d go out and support Jackie but this is a big, big moment for Juliet,” their mother Lisa Evancho shared with ABC News “Nightline” co-anchor Juju Chang. “We’re just going to have her SRS procedures on sex reassignment surgery.”

While their mother will accompany Juliet for the biggest transformation of her life, their father Mike Evancho and the two younger Evancho children will be in Washington, D.C., cheering on Jackie.

“Go big or go home,” said Mike Evancho. “If I sat and maybe thought about it, I would probably be really stressed out.”

Since her sister first came out publicly as transgender, Jackie said she’s been “really excited” for the day when Juliet could undergo the surgery.

“It sucks for me to not be there for her but I'm going through something really big on the same time,” Jackie Evancho said. “So, I guess we both just wish we could be there for each other. It's unfortunate it's on the same time but we were there in spirit.”

“Two major points in our lives and we can't be there for each other,” she added, getting teary-eyed. “It does suck sometimes.”

Working out of her Pittsburgh recording studio, Jackie Evancho has received a lot of backlash for agreeing to perform at the inauguration. People assume she’s a Trump supporter, which is something Evancho won’t discuss. But online, some critics have called her decision to perform “career suicide,” especially after so many other artists have said no, including Elton John and Aretha Franklin.

“I pay no attention to it because I'm a 16-year-old,” Evancho said. “I don't vote myself. ... So, it's not really a political thing for me, it's an honor.”

“If you're a true friend or a true fan, then you will take me for me no matter what,” she added. “And you should know my reasoning behind it or why I did this, so I only want true fans.”

But parts of the criticism stems from the Trump team’s gay rights record, and Jackie Evancho said she’s not naive about some of the hate she has been getting online, both about her inauguration performance and about her sister.

“I am doing this for my country,” she said of performing at the inauguration. “I support my sister 100 percent and I think it's a shame that this has people questioning that. It's really nothing to do with it.”

And these hate-filled attacks, she said, often get personal.

“[Some] just say really awful things about my sister, who she is and what she is and how my parents handle me, because they have no idea what's going on,” Evancho said.

Born “Jacob,” Juliet Evancho is an artist at heart, who has blossomed into a civil rights activist.

“Right now, I'm already receiving backlash from both sides of what my sister has decided to do,” Juliet Evancho said. “Jackie is singing for the inauguration and ... I'm really proud of her for it, because it is a huge thing. You're singing in front of the world.”

Singing for a head of state is an honor Jackie has had before. She has serenaded the pope, the emperor of Japan and President Obama and First Lady Michelle Obama twice.

“Now it's become a new goal for of mine to perform for as many presidents possible,” she said. “That would be cool.”

It’s a lofty goal, one that began for Evancho at age 10, when her audition tape for “America’s Got Talent” went viral on YouTube and lead to a meteoric rise on the singing competition show.

“Most of the time I'm shy, awkward at home,” she said. “When I'm on stage, it's a completely different person and it's nice to step out of my shell and not be worried about it.”

Evancho was crowned the runner-up that season, and won the hearts of many loyal fans. Six years later, she has released seven albums, earning gold and platinum records in classical music along the way.

Her sister Juliet hopes to model one day. This fall, she made homecoming court at Pine-Richland High School, where her sister Jackie is a sophomore.

“Everything has been a competition ever since we were little,” Juliet Evancho said. “I actually wanted to be a singer before Jackie did.”

Sibling rivalry for sure, but Juliet said they have always been devoted sisters, even when she was known as “Jacob.”

“When I would play ... make-believe games with my sister, I was always a girl and I would always dress up in the Barbie dresses with my sister,” Juliet said.

There were other moments, too. Just before then-13-year-old Jacob was about to sing a duet with his sister Jackie on the 2012 PBS special, “Music of the Movies,” the producers said they had a problem with Jacob’s hair.

“They were like, ‘Oh, your hair looks all shaggy.’ They’re like, ‘We’re going to cut it off,’ so they cut my hair backstage,” Juliet Evancho recalled. “On the inside it was kind of killing me, because I wasn’t a boy and I was singing the boy role in the song and it just didn’t feel right.”

It would be years before Juliet summoned the courage to come out publicly as transgender. She came out at age 17, thanks to the support of her family.

“Every now and again, you as a parent go and re-watch old movies and stuff like that. Juliet was always Juliet,” Mike Evancho said. “And it was me that was trying to bury that, and saying, ‘Oh, it's a phase he'll grow out of it,’ and now I realize that was not the case. Now, that doesn't mean that when Juliet told me, it didn't hurt, because Juliet is our first-born, was my first son.”

“You mourn the loss of a son,” he added. “And then you can also look at it from a different perspective, which is what I kind of did, which is, I can either mourn a son, or I can take comfort in the fact that I still have a child, which is what I chose to do.”

As a transgender advocate, Juliet Evancho recently joined forces with Lambda Legal, the country’s oldest and largest legal organization working for the civil rights of lesbians, gay men and people with HIV/AIDS, to file a lawsuit against her local school board in suburban Pittsburgh after they voted to ban transgender students from using the bathrooms in line with their gender identity.

“After the board had let us be discriminated against, it invited even more discrimination and I was called some pretty awful things,” Juliet said. “People would mumble it as they walked past me and I remember last year ... trash was thrown at me.”

It’s the type of anti-LGBTQ law that Vice President-elect Mike Pence supported when he was governor of Indiana.

“It’s a big deal because a lot of us, we have been living a lie for so long that even the little things mean a lot to us,” Juliet said.

Jackie Evancho made a music video in tribute to their sisterhood, set to the Ed Sheeran song, “All of the Stars.”

“It was a tribute to my sister and for kind of everyone out there who's going through the same thing,” Jackie said. “Whether it's that they're transgender or just they just need help coming out or something and kind of just a tribute to them and in honor of my sister.”

Within the very close-knit family, their father said they support their children “no matter what the situation is.”

“Juliet's transgender,” Mike Evancho said. “We support her 100 percent. Where we see an injustice based on our beliefs, we’re going to go after it. It doesn't matter who the president is or the administration, again we do what's right for us.”

And despite not being together for each of their big moments this week, their sisterly bond is unbreakable.

“She is a beautiful singer,” Juliet Evancho said. “She is a beautiful human being and that is the way I look at it. She is doing something for our country and singing for an inauguration of a president.”