"The big sis love is real #river&remy," she captioned the snapshot on Twitter. The photo sees River look down at her baby brother with an adoring expression.
Clarkson followed up with another picture of Remy. She shared a shot on Instagram of the infant laughing, writing, "This pic was literally taken right after #remy got a shot today #wemakeunicornbabies #RemyBaintworried."
Clarkson and Blackstock married in 2013 and welcomed River the next year.
The singer cited a happy marriage and family as her biggest goal in a 2015 interview with Elle magazine.
"I think, especially in this society today, it's really hard to have a successful marriage," she explained.
"Especially for people in the public eye. It's hard to have a successful marriage and be successful as a parent while wanting to have a career."
"I just want to be the best mom ever and I just want to be successful at being a wife. Always be loving and supporting. It's all very selfless," the star said. "I'm 32. It's time for that."
Clarkson came to fame by winning the first season of "American Idol" and has since released seven studio albums.
She last released Piece by Piece in 2015, which debuted at No. 1 on the Billboard 200.
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