Though transgender people are increasingly in the news, Nyr and Thomas represent an almost unheard-of phenomenon - transgender siblings who are not twins. This case is an anomaly, says Norman P. Spack, a pediatric endocrinologist who has treated more than 300 transgender adolescents. Even among transgender people with an identical twin, both siblings are transgender only 20 percent of the time.
In many ways, Nyr and Thomas are like other transgender adolescents. They have both felt depressed and suicidal, as do four in 10 of their transgender peers. Little wonder that seeing Nyr joyful in her purple dress made her brother so happy.
Both worry about their future, and whether they will find jobs, families, and acceptance.
Some transgender people opt for hormones or surgery, others do not, and that's a diversity the siblings share. Thomas is comfortable with his body as it is, complete with monthly menstrual periods. Nyr is using a medication that blocks the early signs of puberty caused by hormone changes. Thomas envisions his future with a male partner; Nyr says she is pansexual - a partner's gender won't matter to her.
Though most transgender adolescents say they knew from a very young age that their bodies didn't match their gender identity, Thomas says he did not recognize he was really male until he reached his teen years; Nyr says she knew for sure at age 10.
Thomas says that, growing up, he always felt somewhere in the middle.
"There were days when I was 6 or 7 where I would have what I called 'boy days.' I had a favorite boy outfit, like something a farmer would wear - baggy pants and a big checked cotton shirt. I felt stronger, more robust on those days. I played with other girls, but I knew I wasn't like them. I hated shopping and I didn't like princesses or dolls. I felt isolated."
When he was 14, Thomas saw a video on transgender people. "It resonated for me," he says. "For a long time, I had pretended to be a boy. I just didn't know I was one."
A video also was the trigger for Nyr. "I think it's always been there for me," she says, "but I didn't recognize it until I was watching a video with a transgender character. "Something clicked, and I thought, 'Oh, yes, I'm like that.' "
Parents' struggle to accept
Thomas and Nyr grew up hopscotching between their home in Springfield, Delaware County, where their father managed a 24-hour service station, and their grandfather's place in Montpellier, France, where their mother, Emma, was raised. Emma, then a freelance journalist and photographer, met her future husband, Cesar, in 1994 in Paris, when he was traveling from his native Mexico.
They moved to the United States a couple of years later and have lived in Pennsylvania full-time since 1999, with the exception of a two-year period in France after Emma's mother died. While there, Emma homeschooled Nyr; her father, a college professor, gave Thomas private lessons in history, Latin, and Greek.
Emma now "unschools" her children, a variation of homeschooling in which the students play a major role in determining each day what they will do. Both children are intense readers, deeply interested in computer technology, and fluent in English and French. Thomas also is learning Spanish.
Emma and Cesar Medina-Castrejon are separated, but both say their conflicts simmered long before the children's transgender issues surfaced. They learned at different times that their children were transgender.
"Thomas was never a traditional girl," said his mother, a soft-spoken, gentle woman with a charming French accent. Emma, who describes herself as "not very feminine," said she was not shocked that her eldest child declared he was transgender.
"Thomas gave me many clues that something was going on and I could see that it wasn't going away," his mother said. "So when he insisted on cutting his hair short and confided, 'I want to look more like a boy,' I wasn't surprised. I took the time to digest it, to educate myself and change the way I spoke and felt . . .. I told him, 'Nothing has changed. You still like animals and reading and folk music and eating steak. It's still you.' "
But she was less willing to acknowledge that Nyr, too, was transgender.
"When she first told me, I thought it was just a game, maybe that she was following her brother," Emma said. "I didn't want to believe it because it was so scary. You hear about how much rejection there is out in the world. You hear about so many being murdered. That's not what you want for your child."
Nyr was insistent.
"So after a while, I had to accept that this was for real," her mother said.
Emma took Nyr to the Gender and Sexuality Development Clinic at Children's Hospital of Philadelphia. Linda Hawkins, the clinic's co-director, says that in the last two years, 250 families with children who say they are transgender have come to the clinic.
"They told me that my daughter was not just going through a phase," Emma said.
Both children say they have never felt as close to their father as to their mother. Cesar was in France three years ago visiting his father-in-law when he received an email from Thomas.
"I have a feeling you won't like the contents of this letter," Thomas wrote. "But I'm telling you because I'm tired of hiding who I truly am from you. I think it's obvious that I'm not your stereotypical girl. I don't wear skirts or dresses or makeup. But I'm not just a masculine girl. I'm not a girl who wants to be a boy. I'm not even a girl who thinks he's a boy. I'm not confused. I AM a boy."
A year later, Nyr told her father she was a girl and wanted to start taking hormone-blocking medication.
"It was the ultimate shock," said Cesar, who now lives in Texas. "I'm very traditional and my logic is a fixed idea of what is a man and what is a woman. But regardless of what I feel, this is not about me. It is about my children and the way they choose to live their lives.
"But I don't pretend to understand it. What are the chances of having two children like that?"
Experts in genetics and transgender issues can't answer Cesar's question.
"It could be random chance or it could be a family dynamic that conveys something to children," said Michael Bailey, professor of psychology at Northwestern University. "So when the first one transitioned, the other got the idea. Or there could be something genetic going on."
Emma, too, wonders about genetics. She recalls a great-aunt who married, had children, and later revealed she was transgender. Everyone in the family except Emma's father shunned her.
Transgender community growing
The adult transgender population in this country is about 700,000, estimates Gary Gates, a demographer at UCLA's Williams Institute, a national think tank for research on gender identity.
Kim Pearson, director of training for TransYouth Family Allies, an Internet support group, says she is startled by her organization's growth from 15 families in 2007 to nearly 500 now.
Carrie Jacobs, executive director of the Attic, a Philadelphia LGBT youth center, says she believes the transgender community is much larger than such figures suggest.
"Many children are still hiding their identity from family and friends," she said. "They fear being shunned or worse - bullied, violated, and maybe even killed."
At least one in 1,000 children and adolescents may be transgender, estimates Spack, who in 2007 established the first-of-its-kind Gender Management Service Clinic at Boston Children's Hospital.
Spack thinks the roots of gender identity are to be found in fetal development. During the early weeks of pregnancy, the fetus - which always begins asexual - will develop a male or female sexual anatomy. In the second trimester, the brain becomes masculinized or feminized. The umbilical cord, blood, placenta, and amniotic fluid release hormones and chemicals that influence gender in the brain, he explained.
"Usually, sexual anatomy and gender identity match, but not always," he said. "Or it may be that there is something intrinsically different in the brain of someone who is transgender. The brain is a black box and we just don't know the answer yet."
Acceptance doesn't mean easy journey
Most experts advise parents such as Emma and Cesar to support their children rather than deny that they are transgender. They urge them to seek professional psychiatric and medical help.
But acceptance doesn't mean the journey will be easy.
Once she recognized she was a girl, Nyr became increasingly depressed, told her mother she felt empty, and talked about death. She was terrified of the changes puberty would bring, particularly facial hair and a deepening voice. She struggled to get her father's permission to start puberty-blocking drugs.
He finally agreed 10 months ago and she began treatment at the Mazzoni Center, the Philadelphia LGBT health clinic.
Nyr says she feels "lighter" knowing that she will be able to have the body that matches her sense of herself.
She hates to take showers because she can't tolerate seeing her naked body. Her brother has heard other transgender teens admit they shower in their bathing suits with the lights off.
But emotionally, her mother said, Nyr is a different child.
"Taking the puberty blockers has made a life-death difference for Nyr," Emma said. "If we stopped them, I believe she would become suicidal again."
Thomas has a completely different perspective on his body, seeing his breasts and vagina as "just another variation of the male body." As for starting his menstrual periods at age 11, "I was actually very excited, because it meant I was growing up . . . and growing up is good.
"Many trans guys will bind their chests, but I think it's too painful. I don't feel like I'm a freak. I'm definitely a boy."
No typical day
There is no typical day in the Medina-Castrejon household. Therapy, most of which the family gets from the experts at the Mazzoni Center, is a big part of their lives. Thomas also participates in a group of other transgender teens at the Attic.
Each week, the siblings meet with other children in families that follow their "unschooling" philosophy of education and share in an activity - perhaps a parent with expertise in dance arranges a field trip to the Annenberg Center to see a dance troupe and meet with the performers.
Like most others their age, they spend a lot of time online with friends. Nyr is writing a novel with some of them, who submit chapters for her to edit. She plays the ukulele and is learning to compose music on the computer. Thomas is starting to think about college and is working on his math skills.
Both siblings say they are grateful, especially to have each other and for their mother's support. They hope for a better rapport with their father, who struggles to make his feelings clear to them.
"I deeply love my children and we have a link for life," he said. "I want them to feel that I'm available to them, that they can count on me, that they have my full support and protection."
The future? "I think about some day having a male partner and a family - maybe two children," Thomas mused.
"Me? I want to be an artist," Nyr said softly. "I want to make music."
Gloria Hochman is a Philadelphia journalist who writes frequently about transgender children and other medical and social issues.