Jenna Bush Shares Birthday Story About Twin 'Sissy' Barbara Bush: 'She's Always Made Me Feel More Courageous'

Jenna Bush Hager turned 38 on Monday and celebrated with a touching story about the person who has been beside her for every one of those years: twin sister Barbara Bush.

“Another trip around the sun with my sissy!!!” the Today show co-host wrote about her Sisters First co-author in an Instagram post along with three pictures of them together: one as girls, laughing and playing, and two as adults.

The daughters of former President George W. Bush and former First Lady Laura Bush, Hager and Barbara have recently been on tour with their latest book, a childhood story about two sisters who forge their own strong bond. (That book shares a title with their memoir, published in 2017.)

In her post on Tuesday, Hager reflected on the devotion she and her sister have for one another.

She wrote about the first time they flew alone on a plane together, when they were 7 years old and visiting their grandparents in Texas.

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“Holding Barbara’s hand as I walked down the tarmac, I felt brave,” Hager wrote. “She’s always made me feel more courageous. And funny. No one laughs at my jokes like she does.”

On the plan, however, they started to argue — until a man seated on the aisle turned to her and said,

“‘Are you going to hit her or should I?’ And that ended our fight” Hager wrote. “We were a team again.”

Sister Barbara, older by one minute, “put her little arm around my shoulder,” Hager continued. “No one was going to hurt her sister. And that’s my sister: all heart and all action. She loves fiercely and lives joyfully. And, I’m so lucky she’s mine! Happy Birthday sissy!”

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The Bush twins have never not been close.

“I was born with a partner to go through everything with,” Barbara recently told PEOPLE. “She’s my reference for the world.”

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Barbara said she cherished her book tours with her sister, saying they got to explore new cities together, have a slumber party and watch a movie at night — as though nothing had changed since they were kids.

“Watching us fall in love with each other, my mom was so thrilled about that,” Hager told PEOPLE in an interview for this week’s issue. “There’s nothing more special than having a twin sister.”