Katherine Schwarzenegger Hugs Daughter Lyla as They Celebrate Her Third Birthday: 'Joy of My Life'

Katherine Schwarzenegger Pratt has a big girl on her hands!

The Good Night, Sister author, 33, celebrated daughter Lyla Maria on her third birthday on Sunday. In photos shared on Instagram, the mom of two hugs her daughters, who wears her long hair in pigtails and has on a birthday hat, as Schwarzenegger Pratt smiles at her.

"My 3 year old girl ! I love her so! Happy birthday to my happiest joy of life. 💕," she captioned the shot.

Maria Shriver commented on the photo, sharing well wishes for her granddaughter.

"Happy happy lyla maria everyone loves you," the proud grandmother wrote. "You are a gift from god. We are so lucky to be able to watch you become you, and what a you, you are — smart funny brave loving 🥰."

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Schwarzenegger Pratt also shares daughter Eloise Christina, 14 months, with husband Chris Pratt, who is also dad to son Jack, who turns 11 later this month, with ex Anna Faris.

On her Instagram Story, the mom of two also shouted out her siblings as she shared a post that celebrates "aunt and uncles who make our kids their own."

"The aunts and uncles who step up, even when they aren't expected to. Those who support, advise, and love unconditionally. Those who spoil, offer comfort, and become a safe space. You are so appreciated," it concluded, as she tagged sister Christina, 31, and brothers Christopher, 25. and Patrick, 29.

The author's dad, Arnold, 75, recently spoke to PEOPLE about how proud he is of his daughter. At the premiere of his new Netflix series Fubar in May, the actor opened up about what it’s been like to watch Schwarzenegger Pratt become a mother.

“It's nice to see your kids having kids themselves and to watch this new dimension,” he told PEOPLE exclusively. “You see them growing up and you see them going to school and getting smart and getting their degrees, getting their jobs and all this, but now seeing them be a parent? It's fantastic. It's really great.”

He went on to say how much he enjoys spending time with Schwarzenegger Pratt and her kids. “I just love when she [Katherine] comes over to my house or when I go over there, but most of the time she comes to my house because I have all the animals," he said. "The grandchildren love the animals. I have a pig so now they want to come over there and feed the pig all the time, hang out with the little pig.”