Maddie Ziegler Celebrates Sister Mackenzie's 18th Birthday: 'My Best Friend for Life'

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LOS ANGELES, CALIFORNIA - MAY 18: Maddie Ziegler, wearing Ralph Lauren, attends "Elle Hollywood Rising" presented by Polo Ralph Lauren and Hulu on May 18, 2022 in Los Angeles, California. (Photo by Matt Winkelmeyer/Getty Images for Elle); LOS ANGELES, CALIFORNIA - DECEMBER 07: Mackenzie Ziegler attends Disney Studios' premiere of "West Side Story" at El Capitan Theatre on December 07, 2021 in Los Angeles, California. (Photo by Matt Winkelmeyer/Getty Images)
LOS ANGELES, CALIFORNIA - MAY 18: Maddie Ziegler, wearing Ralph Lauren, attends "Elle Hollywood Rising" presented by Polo Ralph Lauren and Hulu on May 18, 2022 in Los Angeles, California. (Photo by Matt Winkelmeyer/Getty Images for Elle); LOS ANGELES, CALIFORNIA - DECEMBER 07: Mackenzie Ziegler attends Disney Studios' premiere of "West Side Story" at El Capitan Theatre on December 07, 2021 in Los Angeles, California. (Photo by Matt Winkelmeyer/Getty Images)

Matt Winkelmeyer/Getty; Matt Winkelmeyer/Getty

Maddie Ziegler is showing some sisterly love!

In celebration of sister Mackenzie Ziegler's 18th birthday on Saturday, the Fallout star posted two pictures of them as kids alongside a sweet caption.

"You are truly my best friend for life," Maddie, 19, wrote on Instagram. "I'm the most lucky to have you. I'd be lost otherwise."

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"I love you to the moon and back, happy birthday ❤️," she added.

"Maddie I love you," Mackenzie responded in the comments section, "wish you were here to celebrate with me💓."

The Dance Moms duo recently spoke to E! News about their iHeartRadio podcast, Take 20 with Maddie and Kenzie.

"We definitely feel like people needed to see a more personal, real side of us," Maddie told the outlet. "I think we wanted to let our guards down and show something that wasn't so heavily produced and, rather, just us having a pretty casual conversation."

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Mackenzie also admitted they sometimes get too comfortable when recording. "I think the problem with that is sometimes we have these laughing episodes where we just can't stop," she shared. "There's definitely some parts of the podcast that have to be cut out because we're just, like, laughing so hard."

Maddie added, "That would be so much fun for people to just kind of see a live version of what we do. We're casual and we're usually slumped in our chairs just talking."

"I feel like even when I'm bored in my room, I'm just like, 'Maybe should I annoy Maddie right now,'" Mackenzie continued. "I'll call her just because I'm bored at home."

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In 2019, Mackenzie, who also goes by Kenzie, confirmed to PEOPLE that the two are "super close."

"She's given me so much good advice and is such a big inspiration to me," the "worst thing" singer said of her older sister, who is known for her role in several Sia music videos. "She's doing amazing things. I'm very glad that I have a sister that's so talented that I can look up to and learn from as well."