Identical twins Sienna and Sierra Bernal really are unique - as one is less than half the size of her sister.The 13-year-olds, from Houston, Texas, are the only identical twins in the world in which one is a primordial dwarf.Sienna weighs half as much as Sierra and stands just 4ft tall, compared to her twin's 5ft 1ins height.The blonde-haired blue-eyed teens both love shopping, wearing make-up and giggling about boys.But due to a rare brain abnormality Sienna also has the learning age of an eight year old.Sienna said: "Most of the time I don't really care I'm different - it actually makes me feel special."Sierra looks out for me and we do loads together - shopping, swimming and singing. She can be bossy but she shows me how to flirt with boys, stands up for me if I'm teased and reaches things when they're too high."The sisters were born three months prematurely with Sienna weighing slightly more than a pound.Doctors warned mum Chrissy, 34, Sierra had just a 10% chance of surviving. But after 108 days in intensive care she was fit enough to go home.Chrissy said: "I try to treat the girls as equals. Sienna has a real passion for life and makes everyone laugh. I'm so proud of both my girls."