"How incredible does Kourt look!?" Kardashian, 32, wrote of her sister's unretouched bikini pics. "She has lost 44 lbs in 6 months since giving birth to Penelope and looks better than ever. She is an inspiration to me! While we were in Miami we would always work out together and keep each other motivated and I am so proud of her!"
Kourtney, for her part, told Us Weekly's West Coast Bureau Chief Rebecca Bienstock that she couldn't have done it without her younger sis. "If she wasn't scheduling the workouts, I probably wouldn't have been as motivated," she said. "If one of us isn't in the mood, we push each other."
Now a lean 106 pounds, the mom-of-two -- to Penelope, 6 months, and Mason, 3, with partner Scott Disick -- works out regularly with celeb trainer Tracy Anderson, who is also helping Kourtney's 12-weeks-pregnant sister stay fit.
"Tracy Anderson is keeping me in shape!" the first-time mom-to-be wrote on Jan. 3. "She is helping me make the adjustments necessary to keep me feeling happy, healthy and most importantly create a workout plan that is safe for the baby. It's really important for me to have a fitness routine that works for my body and my schedule and I'm really happy with the workout plan Tracy is working with me on."
For more details on how Kourtney Kardashian got her post-baby body back -- plus her thoughts on marriage and having a third child -- pick up the new issue of Us Weekly, on stands Friday!