03/08/2012 07:21 am ET Updated Mar 08, 2012

'Anderson' Interviews Unique Twins, One Of Whom Is A Primordial Dwarf (VIDEO)

On "Anderson" (Weeknights, 7 p.m. EST on Fox), viewers were introduced to Sienna and Sierra Bernal.

The 13 year-olds are the only known pair of twins in which one (Sierra) is average size and the other (Sienna) is a primordial dwarf.

The girls and their parents talked about their family life and the unique challenges that face someone with primordial dwarfism.

Mom Chrissy said that Sienna lags behind her sister in terms of emotional development, something she thought was caused by the inappropriate ways other people react to her small stature.

Sienna's emotional delay was becoming more noticeable as the girls have gotten older. Although they still dress alike, Sierra said, "She's always wanting to do little kid things, I'm always wanting to do older things."

Standing less than four feet tall, Sienna needs help with many everyday tasks, and even thought she hates it, she still has to ride in a car seat.

Plus, Chrissy said, people tend to treat her like a much younger child. "[They] talk to her like a baby, they pick her up. She's thirteen, she doesn't need to be picked up."

Except, joked stepfather Joe Johnston, by him.

"Anderson" airs weeknights at 7 p.m. EST on Fox.

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