Katherine Schwarzenegger Pratt's daughters call Arnold Schwarzenegger and Maria Shriver 'Opa' and 'Mama G'

Welcome to So Mini Ways, Yahoo Life's parenting series on the joys and challenges of childrearing.

Katherine Schwarzenegger Pratt may be the oldest of Arnold Schwarzenegger and Maria Shriver's four children, but she's always considered her 19-months-younger sister Christina to be the real trailblazer.

"I think everybody expects the oldest to be the one to do everything first and to be the stronger and braver one," the author and mom of two says. "I leaned on Christina to do everything. I turned to her to do everything first: to get her ears pierced first, to go down a slide first, to go to camp first. I was much more timid and reserved as a child and Christina was much more confident and bold. And so I wanted to write a book celebrating the relationship of sisterhood, but also be able to touch on the dynamic between Christina and I growing up."

That book hit stores this week. In Good Night, Sister, illustrated by Lucy Fleming, big sister Kat assures her little sister Tina that all will be fine when she leaves their shared bedroom behind. When a thunderstorm hits, however, it's brave little Tina who emerges as the more confident sibling.

Pratt calls the children's book a "tribute to the sisterly bond" that will also hopefully strike a chord with any kid who has a close sibling relationship or friendship. Though she finished writing the book when she was a new first-time mom to daughter Lyla, now 2 1/2, she and actor husband Chris Pratt are now raising two little sisters of their own. The couple welcomed their second daughter, Eloise Christina, last May.

"I wasn't sure if I was going to have a girl or a boy or when I was going to get pregnant next," she says of writing Good Night, Sister. "And so it's a really special and beautiful surprise that I'm able to have sisters in my own two girls and also come out with a book on sisters and have a sister myself. So it's really exciting and fun — and it's really wild that even at 2 1/2 and 8 months old, you start seeing so many similar behaviors and patterns that my sister and I had."

Pratt's parents "get a real kick" out of seeing the similarities.

"They're like, 'look at them, they're doing the exact same thing that you girls did,'" she says of Schwarzenegger and Shriver. "My parents always show these old videos of me squeezing Christina when I was younger and trying to squeeze her really, really tight and get really excited about it, or wanting things to be mine instead of hers. You know, the normal little sister dynamics. And even though my girls are so young, they have very much so those behaviors and mannerisms coming out already."

In a nod to his Austrian heritage, Schwarzenegger goes by "Opa," which translates to "grandpa." Longtime journalist Shriver, meanwhile, chose "Mama G" as her grandparent name, following the non-traditional example set by her brother, Timothy Shriver, aka "G Daddy." ("They were discussing entering this phase in their life where they are going to be become grandparents, but they so don't feel like grandparents, or, I guess, what they think of as grandparents," Pratt explains.)

Shriver and Schwarzenegger are "incredible grandparents," says Pratt, who is also very close — literally living within minutes of each other — with her siblings.

"My dad's house is really fun because he has all these animals and he starts having [the girls] understand the chores and how to take care of animals and all these different things," she says of the actor and former governor. "He's very sweet and playful with them. So it's been really amazing to watch both of my parents step into the role of grandparents."

In addition to Lyla and Eloise, Pratt is also stepmother to husband Chris's son Jack, from his first marriage to Anna Faris. Watching how the Guardians of the Galaxy star treated his son made her feel confident about building a family with the actor.

"I had the privilege of being able to see how he was as a father before having my girls, with his son," she says. "So I knew he was going to be an amazing dad and I was really excited to see how that was going to play out. And he's really amazing as a girl dad as well."

The Jurassic World star is "a total softie with my girls," she shares.

"He's someone who's sitting in the playroom dressing up with all the princess stuff," she says. "He's definitely very hands on. I think we take different roles with each other as far as like the good cop/bad cop thing. We take turns at it, which I think is important because then they don't associate one parent with having to do everything disciplinarian. We definitely have very open and honest communication about what's important to us, what our values are, how we want to raise our kids."

As a mom, Pratt is big on schedules, especially in terms of naps and consistent bedtime routines. It's a different approach from her own mom, who, despite having four kids, never stuck to schedules. But she is taking a cue from her own upbringing when it comes to screens.

"We're not doing any screen time," she adds, noting her girls' young ages. "I'm a big believer in getting outside, doing crafts, doing things that are outdoors, not sitting inside and being on a screen. That's definitely how I grew up. I was never to allowed to watch TV during the day or even during the week. I think it really allowed us to get creative. And the girls have built-in playmates with each other, so [even if] they reach an age where they wanted to gravitate toward being on a screen, hopefully they'll be able to find a playmate with the other since they're so close in age."

Wellness, parenting, body image and more: Get to know the who behind the hoo with Yahoo Life's newsletter. Sign up here.