Leah Shirley is celebrating her stepsister on her special day.

Amber Portwood and Gary Shirley ’s 16-year-old showed off her bond with her stepsister Karly Anderson , the daughter of Gary's wife Kristina Shirley from a previous relationship, with a heartfelt birthday message.

"HAPPY 18TH BIRTHDAY SISSY!!!" Leah wrote on Instagram Sept. 15 alongside a series of rare family photos. "Remember to have no regrets with wherever life leads you from now on, I love you so much, I hope you had the best day ever."

And beyond the throwback photos, Leah also gave a glimpse into Karly’s 18th birthday bash, which featured a selfie of the dup—with Karly sporting a pink birthday sash.

Leah’s sweet birthday message to her older sister is rare look inside Karly’s private world away from the

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