The former TOWIE star, 22, couldn't hide her disbelief at how quickly time had passed, and was quick to share an adorable photo of her baby sister from years gone by.
Find out the reason why Lucy left TOWIE last series.
"Can't believe my baby sister is 18 today!!!," wrote Lucy next to the snap. "Happy birthday @LydiaMeck... When did u get so grown up."
"@lucy_meck Haha aw thanks Luc!!" replied Lydia.
Proud sister Lucy made a photo montage of her lookalike sister, which shows the A-Level student pictured as a young girl and a more recent image of her modelling Lucy's clothing line.
In her retro snap, a cute Lydia smiles out broadly and looks angelic with her blonde curls scooped up into a half ponytail. The Essex-born schoolgirl has grown into a beautiful young lady and in the second image, Lydia is pictured showcasing a dress from Lucy's prom collection.
Check out more photos of Lucy and Lydia modelling together here.
As owner of Brentwood-based shop Lucy's Boutique, the fashionista is used to modelling her own designs but roped in her pretty younger sister for her latest range. Just last week Lucy shared pictures from the siblings' first photoshoot together.
"I know I'm biased but how beautiful is my sister @LydiaMeck modelling @lucys_boutique Prom," wrote Lucy next to a photo of Lydia in a black sequined dress.
"Modelling the campaign was great fun," said Lydia at the time. "People joke we are like the Kardashians because there are five sisters! Modelling this with Lucy made it feel like we are."
Lucy, who is signed to Select Model Agency, has been keen to get her younger sister involved in the modelling world. The small-screen beauty posted another image on Twitter of Lydia posing in the dresses, which show off Lydia's svelte figure and toned legs.