Sarah Francati was a freshman in high school when she stepped in as her sister Emily’s primary caretaker

When Sarah Francati was 15 years old and her mother was diagnosed with stage-three colon cancer, the teenager didn’t know how to care for a child.

But Emily — Sarah’s younger sister, 10 years her junior, who has Down syndrome — needed a capable parent. Their single mom was sick and incapacitated by chemotherapy and she needed someone to step in and raise Emily. Sarah was as willing as she was unprepared.

“I didn’t have someone who was like, ‘This is what you do, step by step,’” Sarah, now 27, tells PEOPLE over 10 years after she started caring for Emily. “I was like, ‘Well, I know what to do with my other siblings. I’ve done that. Can’t be much different.’”

Their family dynamic was atypical from the start. Sarah, Emily and their middle two siblings are technically half-siblings, as their mother conceived them all using different donors. Despite their age gap, the eldest and youngest daughters were always particularly close.

“I remember telling my mom, ‘Everything is going to be okay. Whether you believe it or not, it will be,’” Sarah tells PEOPLE, recalling her childhood days spent helping out their mother at home. “Then fast-forward, our mom was diagnosed when I was in high school, my freshman year.”

The girls’ mom was given a poor prognosis with a low chance of survival. Her tumor, doctors estimated, had been growing for over 10 years, and she needed extensive surgery and radiation and chemotherapy. She asked Sarah to help raise her younger sister because the future was uncertain.

“She was sick for years, in and out of the hospital,” Sarah says. “Emily and I grew incredibly close. I graduated high school early so I could be home, I could be present.”

Backed by little parenting experience, Sarah cared for Emily with only minimal adjustments based on her Down syndrome.

“If she needed help [it] was available to her, but there were never limitations because mainly I didn’t know,” she says. “I have never looked at her and thought to myself, ‘Oh, I need to hide her away. Oh, I need to infantilize her. I need to treat her like a child.’”

Sarah fully immersed herself into her maternal role, to the point where Emily even addresses her as “mom” sometimes. And when their actual mom’s cancer started to get better, Sarah still didn’t step away from her caretaking responsibilities.

Such dedication proved especially important though, because her mom’s period of remission was punctuated by a series of five strokes. While she’s technically in remission and is now able to help look after Emily at their home in Rochester, New York, Sarah maintains that her mom is forever impacted by illness.

“I tell people to this day: yes, she’s in remission and yes, she’s here, but it’s not the woman that raised me. She’s not completely well, and chemo brain is a very real thing,” Sarah explains, though she has no intention of keeping Emily out of their mother’s care.

“I wasn’t going to take her away from Emily. That was never part of the goal,” the former nurse-turned-teacher tells PEOPLE. “I wanted her to be present with her biological mom. There’s study after study showing the importance of that. But for a good 10 years, it was just me and her.”

Sarah notes that she only ever resented her premature role as a parent once, when she was in nursing school during her early 20s. While people her age enjoyed the “college experience,” Emily’s older sister lived off-campus, at home, working to graduate early and commuting back and forth for her classes at nursing school.

“Now I see the other side and I’m like, I don’t regret that at all. I didn’t need it… I was given such a greater job,” she says.

Years after college, Sarah learned something that made her see her family situation as even more than that, too. After extensive fertility testing, she found out that she is unable to conceive biological children. Such a diagnosis, Sarah knows, can make women feel “incredibly lonely,” but she feels lucky to have experienced motherhood regardless.

“There’s other ways to feel like a mom,” she says. “Even though that’s not the ‘title,’ I’ve been so lucky to have the opportunity to feel like one for 18 years.”

Today, as now-18-year-old Emily finishes high school and prepares to leave for college, the proud big sister can appreciate how her work paid off.

“She knows that she has Down syndrome, she’s very aware of her disability, but she’s hyper independent. I mean, she functions like a teenager,” Sarah says, adding, “I think that’s how every person should be able to have a life. Disability or not, they should know that they have opportunities.”

The sisters have shared their story online in a number of viral TikTok videos, and their online community has created opportunities for the duo to step onto a larger platform. In 2022, Sarah and Emily walked the runway at New York Fashion Week, modeling clothes by Jacqueline City Apparel.

The event marked a milestone Sarah will never forget. She could see clearly that their unconventional dynamic hadn’t hindered Emily’s life at all.

“I realized it didn’t matter if I gave her juice instead of formula. It didn’t matter if I put two diapers on her instead of one. It didn’t matter if I put her in boy clothes,” Sarah reflects, looking back on their moment on the Fashion Week catwalk. “She’s here. She’s thriving. We did it.”