Ava Phillippe Calls 13-Year Age Gap with Brother Tennessee the 'Greatest Gift I Never Saw Coming'

Ava Phillippe/Instagram

Ava Phillippe loves being a big sister.

On Monday, the 23-year-old daughter of Reese Witherspoon shared a sweet tribute on Instagram to her younger brother Tennessee, whom Witherspoon shares with husband Jim Toth, as he celebrated his 10th birthday.

In the heartfelt post, Ava reflected on the 13-year age gap she shares with Tennessee, revealing that while it "once worried" her, she now finds it to be the "greatest gift." Witherspoon shares daughter Ava and son Deacon, 18, with ex Ryan Phillippe.

"Today's the day…TENN IS 1️⃣0️⃣!! 🎊 Happy happy double-digit birthday to the absolute legend & tiny teenager that is my baby brother. He asks all the best questions, has all the smart (& sassy) comebacks, and shows up in life as nothing less than his most authentic, goofy, and feisty self," Ava began the lengthy caption. "He works hard and plays even harder and is constantly surprising those in his life with his immense maturity, intellect, and compassion for those he loves."

"I once worried about our 13-year age gap, getting to share some of my favorite childhood movies, shows, books, activities, and memories with him has been the greatest gift I never saw coming," she continued. "Thanks T for making me an even better big sister & for all that you teach us just by being you. ❤️ Pava"

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RELATED: Reese Witherspoon Celebrates Son Tennessee's 10th Birthday: 'So Lucky to Be Your Mama'

The Legally Blonde star, 46, also paid tribute to her little boy with a sweet post on Instagram Monday.

Witherspoon shared a cute photo of a smiling Tennessee wearing a pair of oversized black sunglasses and a blue polo with a starry background.

"Happy 10th Birthday to my wonderful boy, Tennessee!! Your brilliant curiosity and endless energy make me smile every day! I'm so lucky to be your mama. I love you buddy! 💙⚽️⛳️🏈🌟 Tenn turns 10!" the actress wrote.

Witherspoon also included a heartwarming picture of the pair on a beach together as she hugged her son from behind and rested her chin on his head. "Happy birthday to my sweet boy!" she added.

Earlier this month, The Morning Show star celebrated daughter Ava with an Instagram post in honor of her 23rd birthday. The actress posted a beautiful photo of the look-alike mother-daughter pair posing in front of a beach sunset.

"Happy Birthday to my favorite daughter, Ava !! 🥰Every day that I get to watch you become the most amazing woman is such a gift," Witherspoon wrote. "Your gigantic heart, generous spirit, and endless creativity inspire me every day. I love you so much 💞💫🎂."