The international supermodel, who captioned her picture "I LOVE ATTICUS! #whydoistilllooklikeababy", can be seen staring up at the camera while feeding the two-week-old baby.
Atticus is her first nephew, and is the son of her older sister Chloe and husband Edward Grant.
Edward also showed his followers several pictures of Cara feeding Atticus, with one of Saturday reading: "Aunty Cara feeds me so well @caradelevingne", while another on Sunday was captioned: "So much aunt love this weekend"
Edward, who Chloe married earlier this year, confirmed their happy news by posting a picture of their new bundle of joy on Instagram on 11 June.
"Atticus Ned Delevingne Grant born last night after 18 hours - @cdelevingne was simply incredible and produced the cutest little boy xxx," he wrote alongside the cute snap of the newborn baby.
It's been an exciting year for the Delevingne sisters. Chloe and Edward married in February, with sisters Poppy and Cara acting as bridesmaids for their eldest sibling.
Poppy was the next to walk down the aisle, tying the knot with her beau James Cook.
She had two ceremonies to celebrate her marriage. She officially said 'I do' during a romantic church ceremony at St Paul's Cathedral in Knightsbridge, West London.
HELLO! magazine had exclusive pictures of Poppy in her Karl Lagerfeld-designed gown. Having Chanel make her dress was "a real pinch me moment", she told us.
The second part of their marriage celebration was held in Marrakesh a week later.