20-year-olds Sierra and Sienna Bernal, of Tomball, Texas, are identical twin sisters but were born with a significant difference: Sienna has Primordial Dwarfism. The condition causes small body size, resulting in Sienna being more than a foot smaller than Sierra, and weighing only 50 pounds, in comparison to Sierra’s 98 pounds. Sierra told Barcroft Studios: “Sometimes whenever people find out we're twins they’ll ask ‘how old is she?’ So a lot of times even after you've established that we’re twins they'll ask Sienna if she's younger. I’m like ‘that’s how twins work, we’re the exact same age’. And even sometimes people think she’s my daughter.” Sienna, who weighed just one pound seven ounces at birth, added: “People do often think I'm younger than my age. They think Sierra is my older sister and they treat me differently. When they talk to me like a kid they'll be like ‘how are you?’ I’m like ‘I am fine. I am 20 years old’.” The girls say they might be the only instance in the world of identical twin sisters where one has Primordial Dwarfism. Sienna was also born with Dandy Walker syndrome, which causes abnormal brain growth. Despite these major physical differences, the sisters are just as close as any other identical twins, sharing a unique bond and supporting each other through everything. Now Sienna is writing a book, and recently started college while Sierra is an accomplished singer and songwriter. Mum, Chrissy, said: “They have the sibling bond, like normal siblings do, but with being twins, they're definitely closer. You can see it if they've been separated during the day, they will immediately come together, and you can tell they really did not like not being together.” In the future, Sierra hopes to continue her success in the music industry, while Sienna is looking to graduate college, and publish a book.