Heidi, who starred in the MTV reality series alongside Holly, posted the snap on Instagram (22 May) yesterday for 'Throwback Thursday'.
The sweet photo shows Holly holding their pet dog in one arm and sharing the pooch's lead with Heidi. Too cute.
Alongside the photograph, Heidi commented: "#TBT with my sweet sister @halzbalz!"
Heidi first found fame in MTV's Laguna Beach: The Real O.C. as Lauren Conrad's friend, then later went on to have a lead role in spin-off show The Hills.
Holly, meanwhile, landed a supporting role in The Hills in 2008 and often appeared on the show. Famously, she threw a potato at Heidi and husband-to-be Spencer during their wedding rehearsal dinner!