Kim and Kourtney Kardashian have a true-blue bond.

Kim took to Instagram Monday to wish older sister Kourtney a happy 43rd birthday, posting a series of photos of the pair twinning in cobalt blue swimsuits to celebrate.

The two siblings matched in pieces from Kim’s new Skims Swim line, which was released last month and almost immediately sold out.

The birthday girl sported a scoop-neck one-piece ($88), while the brand’s founder wore a tank-style bikini top ($48) and swim shorts ($42).

“Each year around the sun with you I learn and grow and evolve more because of you! Thank you for teaching me to always go for it and follow my heart even if no one else understands. They just aren’t on our growth path lol,” Kim wrote to Kourtney.

“Happy Birthday @kourtneykardash! Happiness looks so good on you. I love you so much!!!”

“I love you so much! We keep growing and evolving and learning together!” Kourt wrote back, adding blue heart emojis.

Plenty of other friends and family members wished the eldest Kardashian sister well on her big day, including Kris, Kendall and Kylie Jenner and both Khloé and Rob Kardashian.

Fiancé Travis Barker posted a sultry black-and-white snap, writing, “My best friend, my lover, my everything. Happy Birthday @kourtneykardash I love you,” to which she responded, “All I could ever dream of and more.”