25-Year-Old with Dwarfism Holds 2 Twin World Records with Identical Sister Who Is Over a Foot Taller (Exclusive)

Sienna and Sierra Bernal hold two world records, but they tell PEOPLE it's kind of funny to be recognized just for existing

Growing up, Sierra Bernal was captivated by Guinness World Records.

“I would always read records and be like, ‘Oh, these are so cool,’ ” the 25-year-old aspiring singer and songwriter tells PEOPLE. “But I never really thought that I would actually ever have one.”

Well, as it turns out, she doesn't have just one, she was two.

Sierra and her identical twin sister Sienna set the record for rarest type of discordant twinning and sharing the greatest height differential between living identical twins — and they're both featured in this year's special edition to celebrate the 70th anniversary of Guinness World Records.

Sienna, or “Sinny,” was born with a form of primordial dwarfism that is so uncommon it has yet to be formally classified, according to the record keeping organization. Sinny stands at 4 feet, 4 inches, while her sister is 5 feet, 7 inches, giving them a height differential of over a foot.

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Nearly 26 years ago, their mother, Chrissy Bernal, gave birth to the girls prematurely on Dec. 22, 1998.

Sinny weighed just under 1 lb., 8 oz., while Sierra weighed 4 lb. 4.8 oz. Sinny was later diagnosed with primordial dwarfism, a rare condition that fewer than 200 people worldwide have, their mother tells PEOPLE.

Her mom goes on to explain that the genetic change "happened after the egg split, which is how they ended up being identical twins with such differences in their DNA."

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A sonogram before their birth showed Sinny had a hole in heart and she was diagnosed with Dandy-Walker syndrome, a rare congenital disorder that affects brain development.

After they were born, however, doctors thought her low birthweight was due to those complications, Chrissy recalls.

“They kept saying, ‘Oh, she's little because of the hole in her heart. Oh, she's little because her lungs aren't developed,’ ” she tells PEOPLE.

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But after a feeding tube failed to have a positive impact on Sinny’s growth, the family sought out the input of a geneticist when the girls were almost 8, which was when they learned Sinny was born with primordial dwarfism.

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Sinny currently lives in Tomball, Texas, where she and Chrissy work as advocates, challenging public places to accommodate people of restricted growth. Meanwhile, Sierra has moved to Nashville to follow her dreams.

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If you ask Sinny, the fact that they became world record holders just for being themselves is kind of silly.

"I think it's funny," she says. "We are just living our lives."

“I still don't necessarily know if I deserved to have the record, because like Sienna said, I am just kind of alive," Sierra adds, "But it is really cool.”