The Sweet Magnolias star was one of the first to like her sister's baby news when she posted on Instagram on Monday.
What's more, Britney's news was announced on Jamie Lynn's daughter Ivey's fourth birthday.
VIDEO: Jamie Lynn Spears opens up about her feud with sister Britney Spears
The proud mom shared a number of photos throughout the day, including many tribute messages to her youngest child.
So far, the rest of the Toxic hitmaker's family have stayed silent. Britney is reportedly expecting her third baby, her first with fiancé Sam Asghari.
Jamie Lynn Spears liked Britney Spears' pregnancy announcement
The award-winning singer shared a cryptic message talking about her fluctuating weight, and what initially turned out to be a message of body positivity turned into a pregnancy reveal.
She wrote: "I lost so much weight to go on my Maui trip only to gain it back … I thought 'Geez … what happened to my stomach???' My husband said 'No you're food pregnant silly!!!'
Jamie Lynn and Britney are currently not speaking
"So I got a pregnancy test…and uhhhhh well…I am having a baby… 4 days later I got a little more food pregnant. It's growing!!! If 2 are in there … I might just lose it…"
She continued: "I obviously won’t be going out as much due to the paps getting their money shot of me like they unfortunately already have … it's hard because when I was pregnant I had perinatal depression… I have to say it is absolutely horrible…
This is Britney's first child with fiancé Sam Asghari
"Women didn't talk about it back then … some people considered it dangerous if a woman complained like that with a baby inside her… but now women talk about it everyday…
"Thank Jesus we don't have to keep that pain a reserved proper secret… This time I will be doing yoga every day!!! Spreading lots of joy and love!!!"
The 40-year-old is already a mother to two sons, Sean, 16, and Jayden, 15, from her marriage to Kevin Federline.
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