The star says her daughters adore their baby brother - up to a point!

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Credit: Jessica Alba/Instagram

Jessica Alba and Cash Warren welcomed son Hayes eight months ago, but there was almost no adjustment period for big sisters Honor, 10, and Haven, 7: they just jumped right in.

“It’s weirdly been pretty awesome,” Alba says when asked if there were any growing pains adding a third to the family. “If anything, I feel like it brought us together in the best way and we’re closer than we ever thought … I never thought we weren’t close, but it just really tied us all together in a really sweet way.”

It helps, of course, that her daughters are so excited to take on responsibilities for their so-cute-he’s-a-diaper-model baby brother. “They’re at the age when they’re playing with American Girl dolls and it’s like a real life American Girl doll. It’s kind of like the perfect age, age 7 and 10, to have a little baby around,” Alba tells PEOPLE while discussing her Honest Company‘s upcoming collaboration with Sugarfina (for which Hayes will be modeling, of course!).

“They like to feed him. They like to play with him. They like to pick him up. They like to sing to him, give him a bath. They like to do all the things,” Alba says – with one exception: “Diaper changing is probably the only thing they don’t like to do.”

RELATED VIDEO: Jessica Alba Breastfeeds Son Hayes in a Target Dressing Room

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And she’s happy to have the extra hands. With increasingly busy schedules for the whole family (the girls are back to school, her husband’s hard at work on his Pair of Thieves sock line and Alba’s filming L.A.’s Finest in addition to her work with Honest) Alba says her parenting style is more relaxed than ever with one exception: She’s rigid about making sure all her kids are happy and healthy. “If I see any of that being compromised, then we stop everything and we go in and fix the situation. If everyone’s happy and healthy … the rest of our life is pretty flexible,” she says. “We really try to do Friday night family time with the kids where we make dinner or order in and watch a movie all together as a family. We play games. That’s an important time. We just like to stay at home and try not to work, and try not to be on our phones on the weekends.”