The reality TV star - who shot to fame on MTV's The Hills - posted the pictures on Instagram yesterday (10 February) in honour of her sibling turning 21 years old.
She wrote on Twitter: "Happy 21st Birthday to my little brother Sky!!!!! Love you so much! Seems like yesterday you came home from the hospital! 21 blessed years!"
And we have to admit, she looks almost unrecognisable cuddling up to her bro in these snaps.
In one of the throwback images, Heidi is seen wearing a colourful jumper and flashing her braces at the camera, while Sky gives her a huge hug.
The second picture shows the star with dark blonde-brown hair and sporting a fringe, as she hold little Sky in her arms.
Looking at that innocent smile, it's hard to imagine that Heidi would go on to become one of the most talked about and slightly controversial cast members on The Hills years later!
During her stint on the popular MTV series, the 27-year-old famously married bad boy Spencer Pratt and also revealed that she'd had ten cosmetic surgery procedures in one day, including rhinoplasty and a boob job.
She has since spoken openly about how much she regrets her surgery.
During an interview with Reveal at the start of the year, Heidi admitted that changing the way she looks was her "biggest regret".
She added: "I've now corrected the surgery and reversed some of this damage I've done to my body."
Although it might be a tough reminder of her appearance before going under the knife, the blonde Californian doesn't mind seeing old photos of herself.
"I like seeing pictures of me before The Hills – I don't mind at all."
That might be another reason for these sweet throwbacks!
What do you think of Heidi's childhood pictures? Let us know below, as the best comment from across the website each week wins £50!
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