"I think our girls might be in school together at the same time — that's so fun and exciting for us to watch," Haylie Duff previously told PEOPLE


Hilary Duff and her big sister Haylie Duff are bringing the cuteness.

The sisters brought their families together on Wednesday to mark the birthday of Hilary’s son Luca Cruz, who turned 7 that day, posting several adorable pictures of their younger children to the delight of their combined 12 million Instagram followers.

After a special celebratory evening, the 31-year-old Younger star wrote alongside a cake-infused video, “Last night was so fun💚So grateful for family. And friends. And wild kids.”

Hilary’s 4-month-old daughter Banks Violet and Haylie’s daughter Lulu Gray, 9 months, looked adorable together in a photo Hilary shared to Instagram, posing on the couch in matching splatter-paint onesies from Little Moon Society.

Credit: Haylie Duff/Instagram

“Baby besties Banks and Lulu 👯‍♀️,” she captioned the cute shot while Haylie wrote to accompany the same image on her own Instagram account, “Lulu & Banks … twinning baby outlaws!”

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Founded by the older Duff sister (who’s also mom to daughter Ryan Ava Erhard, 3½), the Los Angeles-based chic brand markets unique clothing for babies, toddlers and kids. (The exact onesies can be found here, retailing at $52.)

Haylie also posted an image of the two moms and their young daughters together, writing, “@hilaryduff and I always take so many pics of our kids that we never seem to be in them together!”

While their baby girls wore the matching onesies, the sisters donned some coordinating pieces as well. Both Hilary and Haylie wore black knit tops.

“Yesterday we had the best evening! Celebrating the first baby to join our family — our big boy, Luca!” continued Haylie, 34. “I can’t believe he’s already so big. So proud of him!”

RELATED VIDEO: Hilary Duff Is Pregnant! Actress Expecting a “Little Princess” with Boyfriend Matthew Koma

The proud mamas, who were pregnant at the same time, welcomed their daughters (for Haylie, Lulu with fiancé Matt Rosenberg in June, and for Hilary, Banks with boyfriend Matthew Koma in October) less than five months apart.

“I think our girls might be in school together at the same time — that’s so fun and exciting for us to watch. It’s great,” Haylie told PEOPLE in November. “We’re so grateful that everyone is finally here and healthy and happy, so no complaints on our end!”

However, Haylie and Hilary don’t swap parenting tips for the most part — they just focus on being there for each other.

“Everyone has so much advice all the time, so we really just try to support each other whenever we feel like venting or talking something out,” Haylie explained.