The undated snapshot showed Jamie Lynn as a little girl with glasses, laughing with her sibling and Timberlake.
"Happy belated birthday to this beautiful soul!!!" Spears captioned the image Friday, in honor of actress and singer Jamie Lynn's 30th birthday, which was on April 4.
"I love you so much and damn it I wish I was as smart as you at 30!!!! It's weird to say that I was basically your momma when you were younger and yes I'm still older but your soul is and always has been wise !!!! I'm truly and genuinely inspired by your path and the life you have created for your beautiful children and I'm so [expletive] proud to call someone as beautiful inside as you my sister!!!! And not only are you a beautiful soul you're so DAMN STRONG ... I was extremely taken back and still am to this very day!!! ... PS I wish I was as strong as you and happy birthday."
Spears, 39, dated Timberlake in the early 2000s. She has been in a relationship with Sam Asghari since 2016.
Timberlake, 40, has been married to actress Jessica Biel since 2012.
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