Britney Spears has been all over the news lately thanks to her new gig as an "X Factor" judge ... but for sister Jamie Lynn Spears, it's been much more low-key.

Jamie Lynn
kept cool on Memorial Day -- by hitting the pool with adorable daughter Maddie!

The 21-year-old kept a low profile the holiday weekend, tweeting about having "#littlemoments" with her 3-year-old little girl.

While Spears' ex-boyfriend and baby daddy Casey Aldridge was M.I.A., Jamie Lynn recently said they'd always have a close relationship.

"We tried. We really wanted to do it right," she told Glamour earlier this year. "We loved each other. I will love him as Maddie's father until the day I die."

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