Katherine Schwarzenegger on Her Daughters' Loving Relationship with Grandma Maria Shriver (Exclusive)

"My mom and my older daughter, Lyla, try to do everything matching … they are very connected in that way,” the author tells PEOPLE

Katherine Schwarzenegger’s little girls love their grandma!

At the launch party for her latest endeavor, the Cleobella x Katherine Schwarzenegger collection, on Saturday, the author revealed that she got a lot of the inspiration for the line — which features a matching mother-daughter dress and PJ set — from her own family.

Specifically, Katherine, 33, opened up about her daughters’ (Lyla, 3, and Eloise, 17 months) close relationship with their grandma, Maria Shriver.

Related: Katherine Schwarzenegger Shares Photos of Daughters Enjoying Scenic Vacation with Mom Maria Shriver

"I feel like it's always a milestone,” the eldest daughter of Arnold Schwarzenegger tells PEOPLE. “My mom and my older daughter, Lyla, try to do everything matching. So I'm really excited as we go into this holiday season that they have another outfit to be matching in, because they are very connected in that way.”

As for Eloise, the mom of two says she is “just growing so quickly” — “and also trying to copy everything that her big sister does.”

“So these matching outfits make a very easy milestone moment of them also entering into their own personalities, which reminds me a lot of my sister and I as we were growing up,” she says, adding: “also just documenting everything as a mom is fun.”

Related: Katherine Schwarzenegger and Chris Pratt Celebrate Fourth of July 'Done Right' with Their Kids

Shriver, 67, twinned with her daughter at the event in a plaid dress from her new holiday collection. Dressed in an all-black Cleobella frock, Christina Schwarzenegger, 32, also supported her sister at the event, where she complemented both Katherine and their mom.

The sister duo never shed their desire to coordinate outfits, and neither have Lyla and Eloise — at least “not yet,” Katherine says of her little ones who “love to match.”

“But I also look at myself and I'm 33, and I match my mom and my sister still,” she continues. “I feel like my sister and I went through a phase where we were like, ‘We do not want to be matching,’ but that's also why we incorporated different pieces into this collection. So you can still get something for your whole family. And it doesn't have to be exactly matching, but if you do love to match, it works that way too.”

“So I'm about to be 34 and I'm still matching with my sister and my mom,” she concludes, adding: “we have a whole family matching affair.”

Related: Chris Pratt and Katherine Schwarzenegger's Relationship Timeline

Shriver showed her daughter — and her collection — some love on Instagram, with a post dedicated to the launch.

“There’s nothing I love more than getting to celebrate my children’s accomplishments, and this morning we all got to celebrate @katherineschwarzenegger’s amazing new collection with @cleobella,” Shriver wrote alongside a video of her family snapping a photo together at the event.

She continued, “This beautiful collaboration has been a labor of love for her, and she has filled the collection with pieces made to celebrate the season as a family. I am so proud of Katherine and can’t wait to wear each look during the holidays, and throughout the year!"

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Katherine, who shares daughters Lyla and Eloise with husband Chris Pratt, is not shy about her own special relationship with Shriver.

Related: Chris Pratt and Katherine Schwarzenegger Celebrate 4 Years of Marriage: ‘I Love Life with You’

On Mother’s Day, the author shared a heartfelt message for Shriver on Instagram alongside several photos of her mom, daughters and siblings, Christina, Patrick and Christopher.

"My mama,” she began the post. “The best mama and mama g in the universe. I have felt loved, cared for and nurtured every single second by you. What a gift from God! And don't I know it!"

She continued, “I'm in awe of you always. You do it all for us kids, your grandkids and everyone else you mother. To know you is to be mothered by you. I want to be exactly like you. You're my role model in everything, especially when it comes to being a mother. I love you with my whole heart. Happy Mother's Day mama!”

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