"I knew she would be a prettier girl than me," the AGT alum joked, alluding to her older sister's transition

Red carpets might be run-of-the-mill for Jackie Evancho since the 15-year-old vocal wonder placed second on America’s Got Talent five years ago. But when she stepped out to perform at the Global Lyme Alliance’s inaugural gala in New York Thursday, the whole night was full of firsts for the person by her side, her sister Juliet.

First fancy gala, first time posing on a red carpet – and first time sharing with the world that she is transgender.

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“I was so nervous, I was excited – so many emotions. I couldn’t sleep last night,” Juliet, 17, says of preparing for her public coming out. “But when we finally got [here], it was just like, ‘It’s happening.’ ”

As for her red carpet debut:

“I was like, ‘Oh my gosh, there are so many lights, and so many people are yelling!’ ” says Juliet. “But it was really fun. It was a new experience, and the cameras wanted to take pictures of me, not just my sister.”

PEOPLE sat down exclusively with Juliet, her classical singer sister Jackie and father Mike, all of whom opened up about her journey and why she wants to share her experience transitioning from male to female to help others. Says Juliet: “I couldn’t be happier.”

”I Know This Is Me’

Born Jacob to Pittsburgh-area parents Mike and Lisa, Juliet always knew something felt different.

“Ever since I was little, I always played with Barbies. I put on makeup and played dress-up with my sister, to the point where my mom started getting me princess dresses,” she says.

By 13, Juliet and mom Lisa, now 48, did some research on trans people after originally thinking Jacob was gay.

“She was the one who was like, ‘Maybe you’re transgender,’ ” Juliet remembers. “So I did a bunch of research, I was up really late, didn’t get any sleep because I was just so immersed in all of these different stories that people told. A lot of them resonated with me, and that was when I was like, ‘I’m transgender.’ I knew deep in me that this was not me, and I needed to make the change.'”

So for the past three years, Juliet has been going to therapy, working with her therapist on when, Mike, 48, says, was the right time to decide: “Okay, I’m gonna start livin’ the way I am on the inside.”

That moment, when Jacob became Juliet, came on her 17th birthday in May.

“I came out to my family and extended family, and everyone has been so supportive, and it’s kicked off after that,” she says of sitting down with her family, including younger brother Zachary, now 13, and Rachel, now 12.

“It hasn’t been too long. But to me, it feels like it’s been forever because it’s just I know this is me, and finally the rest of the world gets to see that.”

A Strong Support System

Juliet says her mom backed her immediately, making her feel more comfortable and helping her express her femininity, even when she was just 13.

“At the time, I had short hair I was, just, not a pretty person,” says Juliet – who has long, flowing tresses today – with a smile. “She got me a wig, got me some clothes, and it was just a secret for a while.”

After her mother, one of the first people Juliet told was her sister Jackie, who recently performed for Pope Francis during his visit to America.

“I actually cried because what worried me was that she was going to get teased,” Jackie says. “She told me she was transgender, and I was actually very happy for her because she finally found herself, and she can be who she wants to be.”

Adds Jackie, with a laugh: “But at the same time, I was upset because I knew she would be a prettier girl than me, but that’s okay.”

Now that there’s another girl in the house, there’s even more time spent fighting over the bathroom, the sisters joke.

“It’s not so much doing each other’s nails as fighting over the nail polish,” says Jackie. “We’ll do each other’s makeup, and we shop together.”

And Jackie even recorded a song, dedicating it to her sister’s trans journey. In August, she posted a cover of Ed Sheeran‘s “All of the Stars” as a tribute to Juliet, to say “that I accept anyone who wants to be whoever they want to be,” says Jackie. “And transgender is very important to me because of my sister, so I thought, ‘Why not?’ ”

Sisterly bonding aside, transitioning is a process for the entire family and comes with its fair share of conversations and reconciliations.

“It’s almost like stages of mourning because I lost a son – but I gained a daughter,” says Mike, who found out Juliet was transgender around the time she was 14. “My brother put it best: ‘It’s just a skin suit. It’s about the person inside.’ It’s the same person on the inside, and that’s really all that matters.”

Adds her father: “You want happy children. The unhappiness was living as a male, knowing you’re a female inside.”

While Mike and his wife (who could not make it to the gala because she’s battling Lyme disease herself) supported Juliet, their “biggest concern” was the negativity others might project onto her.

“Anybody that says, whether it’s transgender or homosexuality, is a choice? They don’t have their head screwed on tight enough,” says Mike. “It’s human nature to want to take an easy road, so why would someone choose to take the hard road knowing they’re going to face ridicule, knowing they’re not going to be accepted by everybody?”

”We Hope That Her Story Can Help at Least One Person’

Though Juliet’s family has been extremely accepting, she knows that’s not always the case.

“I’ve seen that [where] some of my friends are going through situations where I just feel horrible. I’m really lucky knowing that I have that love and support around me whenever I need it,” she says.

But Juliet and her family are hopeful that by sharing her story, they’ll make a difference.

“We let her know what was gonna happen: Some people were gonna embrace you as an inspiration; others are going to chastise you,” says Mike, who’s proud of his eldest for her bravery.

As it turns out, Juliet’s already encouraging change. This fall, she returned to public school, only this time as a female.

“The school has been very impressed, and they wanted some guidance for potential future transgender students,” says Mike.

Overall, Juliet and her family will be happy exposing their very personal journey even if it just saves one life.

“In the transgender community, the suicide rate is extremely high,” says Mike.