The special bond that twins share is truly a magical thing! And though not all pairs look exactly alike, it’s clear that the experience of sharing a womb can have a profound effect. Take sisters Sienna and Sierra, for instance. These two have quite the physical difference between them, but their friendship is the glue that undeniably brings them together. Here’s their incredible story.

When the two were born 6 weeks premature, Sienna – or “Sinny” as she likes to be called – was much smaller than her sister. Weighing in at just 1 pound, she was also 1.5 months behind in development compared to her twin Sierra.

Things weren’t looking so hot for the tiny twin. According to her mother Chrissy, the doctors only gave the girl about a 10 percent chance of survival, with many feeling that she would be a vegetable if she did live past infancy.

But, as time went on, the girl started to thrive, and she began developing into a bright, energetic kid. Nonetheless, it was clear that she was not growing at anywhere near the rate of her sister. It’s hard to believe by looking at a photo from their younger years that these two are exactly the same age!

Interestingly enough, it wasn’t until she was six-year-old that doctors finally came up with a diagnosis: primordial dwarfism. So, what does it mean to live as a primordial dwarf? Sienna simplifies it by saying, “It’s this thing where it makes you short. She’s [Sierra’s] normal height, I’m non-normal height.”

Of course, there’s a bit more to Sinny’s disease than what she divulged in her interview. It’s a condition where, according to the U.S. National Library of Medicine, the subject only grows to be about 20 to 40 inches tall. Primordial dwarfism also comes with a range of health problems like irregular bone development and blood vessel abnormalities that can lead to a stroke.

Though we’re sure that these looming health problems are frightening to the girl’s family, at this time Sinny seems to be doing quite well. Her height aside, the only hint that something might be awry, is the fact that she wears hearing aids. Otherwise, the twin seems to have enough energy and intelligence to go around!

What makes the girls even more special is the fact that, currently, they are the only set of twins who were born under these specific circumstances—with one being a primordial dwarf and the other not—in the world.

These days, the girls are growing into beautiful young women. Sinny has become a bit of an activist in her own right; she has founded a website that educates people on the disease. She also uses it as a platform to support legislation for equal restroom access for people like her.

To see what the twins look like as pre-teens, be sure to watch ABC News’ video below. We know you’ll just fall in love with Sinny’s outgoing and sweet spirit!

What do you think of Sinny and Sierra’s tale? Do you have primordial dwarfism in your family? If so, did this particular story ring true? Tell us all about your thoughts and experiences in the comments section below!