The singer sisters each have three children

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Cutie cousins!

On Sunday, Jessica Simpson shared an adorable snapshot of her youngest, daughter Birdie Mae, 22 months, curling up with sister Ashlee Simpson Ross' newest addition, son Ziggy Blu, 3 months. "#ZIGGYBLU + #BIRDIEMAE," Jessica, 40, captioned the black-and-white photo of the two babies peering into the camera.

Sisters Jessica and Ashlee, 36, both have three children. The older sibling is also mom to daughter Maxwell "Maxi" Drew, 8½, and son Ace Knute, 7½, whom she shares with husband Eric Johnson. Ashlee also shares 5-year-old daughter Jagger Snow with husband Evan Ross, as well as son Bronx Mowgli, 12, with ex-husband Pete Wentz.

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Jessica told Shape last month that hearing giggles from her three children helps her decompress and enjoy her day. "When I hear my kids cackle, it's the most healing sound," she said. "It's contagious — my whole family starts giggling together, and it's like some form of happy laughing therapy."

The Open Book author also said "family is everything to me" while adding what she hopes to instill in her little ones as they grow up: "More than anything, I want to teach them morals and values and to keep them safe but not afraid."

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Ashlee announced her pregnancy in late April, sharing photos of herself and her husband, who is the son of Diana Ross, holding up a positive pregnancy test on Instagram. Ahead of Ziggy's arrival, she told PEOPLE in August that "it'll be really nice to go back to the newborn phase."

"Newborn babies smell so good. There's nothing like it. And we can't wait for the snuggles!" she said at the time, as Ross added, "I'm so excited for this little one on the way."

"He's going to be amazing with his outfits," said Ross of his new son's future sense of style. "I'm already starting to create pieces. I can't wait to dress him up in top hats and trench coats!"