(CNN) -- One recent afternoon, Brianda Cabrera was helping her younger sister, Diana, with her math homework.
Brianda, 23, is not only the eldest of three siblings, but for the past nine years she has also been their parent.
The Cabreras, including Brianda, Diana, 15 and Jose, 18, are orphans. "My parents are gone. For me, this is the only family I have left," Brianda said from their home in Sandy Springs, Georgia.
Their father, Rafael Cabrera, died 13 years ago in Mexico. Unable to find a job in her home country to support her children, their mother, Alejandra Medrano, decided to move the family to the United States, where she had a sister living in Atlanta.
But Medrano was struck by a car and killed in February 2005. She was 36 years old and Brianda was 14.
The children went to live at their aunt's home, but the situation was less than ideal.
"It was chaotic," Brianda says. "They had four kids of their own and it was three of us. A couple supporting all those kids ... it was very stressful for them."
Brianda began taking odd jobs to support the family. As soon as she turned 18, she decided to move out of her aunt's house with her two siblings. She became a parent and breadwinner even though she was barely making enough money to pay for food, rent and school.
"She's become a second mother and she took on responsibilities she didn't need to, to take care of us," Diana says through tears.
In addition to living alone and being barely able to survive on what Brianda made, the Cabreras also faced another challenge: they were undocumented.
Under the law, the Cabreras fell into a category known as "unaccompanied minors," meaning undocumented immigrant children who have no parents or guardians in the United States. The number of unaccompanied minors in the country has tripled in the past five years, government statistics show.
Last year, the U.S. Border Patrol took more than 24,481 into custody, compared with 8,041 in 2008. The vast majority in 2012 came from Mexico (13,974), but some came from places as far away as India (23), China (16) and Romania (16).
Immigration attorney Jessica Daman says these statistics, compiled by the Border Patrol, may be only a fraction of the total. "Obviously, that's just the children who are apprehended crossing the border; new arrivals in theory, so nationwide, who knows how many we have that weren't undetected, that crossed without detention?"
Daman, who works at Atlanta's Latin American Association, took the case of the Cabreras to court. "We asked that the children be found deprived, under state law, and that she (Brianda) be named their guardian. With that order the youngest two children were eligible to apply for special immigrant juvenile status and a green card."
But what happens to children of undocumented immigrants when their parents or legal guardians die or are somehow missing or unavailable to care for their children? What happens to unaccompanied minors who enter the country illegally and get caught by immigration authorities?
The Customs and Border Protection agency, under the Department of Homeland Security, turns these children over to the custody of the Department of Health and Human Services within 48 hours of their detention. Through the Administration for Children and Families, DHHS cares for the children through a network of providers that offer foster care, group homes, shelters and residential treatment centers.
The average stay in the system is 45 days and, according to DHHS, 88% of children are released to relatives already living in the United States.
For now, the Cabreras can stay together. Brianda is focusing on the younger siblings' welfare and making sure they stay in school and take advantage of the opportunities this country offers.
"She's going to school and I know she expects us to go to school and she's made that possible by doing all these legal things," Diana says.
Brianda is majoring in accounting at Kennesaw State University and working at an accountant's office to support her siblings.
"Family comes first, so that's the way I was raised that before anything else you have to take care of your family," she says. "So I think that was the main thing. Now that my parents are gone I have to take care of my family because that's the only thing I have left."
The fight is not over. Brianda's fate depends on a bill for young immigrants known as "the Dream Act." But the legislation, officially known as Development, Relief, and Education for Alien Minors, has failed to pass multiple times in Congress.
Brianda, however, says she wants to stay "positive." That's the only hope she has of keeping the family together.
And she's embracing that hope.