When Ava Perlin was assigned a passion project in her high school class, she knew she wanted to focus on inclusion and kindness. So she spent the rest of the year working on a children’s book about her little sister Millie, who has Down syndrome.

Ava’s presentation at the Garrison Forest School’s passion project fair, complete with several copies of the final book and a trifold board, was met with hugs and tears. Soon after, the rising senior was invited to read the book for hundreds of kids at the lower school of the Owings Mills campus, too.

“Almost every teacher asked if they could get a copy for their classroom,” said Ava, of Reisterstown. “So that was like the tipping point almost of what made me think that this could be bigger than it is.”

The 17-year-old officially self-published “One in a Millieon” in early June. The book speaks to siblings of those with disabilities — a demographic that often receives far less attention in research circles than people with disabilities themselves or their parents.

“I aimed to combat the idea that curiosity and embarrassment are bad feelings, because it’s natural to be embarrassed sometimes when you’re in the center of attention because of your sibling,” Ava said.

At the end of the day, advocacy is the most important thing, Ava said. The book follows Millie, who has Down syndrome and loves to dance and sing, from the perspective of her protective older sister. Ava created the illustrations for the book with DALL-E, an AI image generator.

“When Ava showed us the book the first time, we cried,” said Lauren Perlin, the sisters’ mother. “Millie adds a level of emotion because I don’t know that people appreciate how much harder it is for a child with a disability to accomplish the same thing a peer that’s typically developing can.”

Nearly 6,000 babies in the United States are born every year with Down syndrome — a genetic condition that causes physical and mental developmental challenges, according to the Centers for Disease Control and Prevention.

The first page of “One in a Millieon” defines Down syndrome as a condition where the “body and brain grow in a different and special way.”

“This is a story about being kind, being supportive, and showing love to others, no matter how alike or different they may be,” the book reads.

Lauren said that the recognition the book has received made Millie, who is 14, a little embarrassed. Asked how she felt when people asked for her autograph, Millie sighed. She also confirmed that Ava was a really good big sister.

Ava and Millie, both older siblings to their younger sister Sadie, who is 12, have always been close. Growing up, Ava walked Millie home from the bus stop every day. This summer, they take laps around the summer camp they have attended together for the past 10 years, talking about their day and watching the sunset. Millie, Ava said, has taught her to see the good in people.

Before elementary school, Ava had never given much thought to having a sibling with Down syndrome. When the sisters began going to school together, that changed.

“Nobody had bad intentions, people were just curious and kind of confused and unaware that people can be different in this way,” Ava said.

As she’s grown older, Ava has become more secure in herself and Millie. All that matters is that they are happy, she said.

“That curiosity can’t be upsetting to us,” Ava said. “We have to be stronger than that just because people want to learn, and they don’t always know how to react in new situations. So really the most important thing there is to teach others.”

At school, Ava is the only person with a sibling who has Down syndrome. Over time, she has developed a community of siblings like her, mostly through Brother and Sister Workshops hosted by Brian Skotko, director of the Down Syndrome Program at Massachusetts General Hospital.

Skotko said the workshops aim to normalize many of the feelings that siblings of people with Down syndrome may experience, and to remind them that they are “not alone.” Many sisters and brothers will feel guilty, for example, that they are able to accomplish life goals that their siblings with Down syndrome might never be able to.

“It’s about recalibrating what’s most important in life,” Skotko said of those with Down syndrome. “Is the most important goal for them to graduate from college, to get married, to have their own car, to have their own house? Or is it something deeper?”

Skotko was excited to see Ava’s book — and also not surprised. He said that most people who have siblings with Down syndrome keep them in mind when choosing future professions, as he did. His younger sister with Down syndrome inspired him to pursue a career studying the disability.

In a similar way, Ava hopes to study the genetics and science behind Down syndrome in the future.

Skotko ordered many copies of “One in a Millieon” to keep at Massachusetts General for when patients and families have questions. The book can be ordered from Amazon, and in the next few months will be available at several branches of Baltimore County Public Library.

“I wish that I had had something like this when I was that age,” Ava said.