Everywhere Melissa Villasenor goes her little sister, Isabella, follows.

The 6- and 2-year-olds share just about everything. They share big personalities. They share a love of being lively and loud. And, they also share something else — they were both born with biliary atresia.

Biliary atresia is a chronic, progressive liver condition that is fatal if left untreated. For most parents, having just one child with this rare, life-threatening disease is overwhelming.

"My husband keeps me grounded," says Andrea Torre, the girls' mom. "He reminds me, 'See how much they fight for their health? We have to fight twice as hard.'"

When Isabella was a baby, she became sick after contracting an infection. She was transferred from a hospital near their home in Rhode Island to Boston Children's Hospital. Although the youngest and just 11 months old at the time, she would be the first of the two girls to receive a liver transplant. "I was so close to losing her," says Andrea.

Boston Children's transplant surgeons Dr. Khashayar Vakili and Dr. Heung Bae Kim performed the life-saving procedure in July of 2015. Less than two years later, the oldest, Melissa, now is waiting for a transplant.

"Honestly, I feel like a roulette wheel, going up and down and around all over the place," Andrea says.

Every other month, Andrea and the girls — along with big brother Andres — make their way to Boston for Isabella's clinic appointments. On this visit, they are playing with pom-poms, giggling and being goofy, punctuated by shrieks of laughter.

"It hurts me seeing them sick," says Andrea. "But then, I see this. I see them playing as if nothing is wrong and I think, if they can do it, so can I."