Erin and Sara Foster Celebrate Favorite Daughter Store Opening with Famous Friends: 'Means a Lot'

Erin Foster and Sara Foster get by with a little help from their famous friends!

The sisters celebrated their first-ever standalone boutique in Beverly Hills for their clothing line Favorite Daughter, surrounded by a slew of superstars including Kate Hudson, Rachel Zoe, Molly Shannon and jeweler Jennifer Meyer. And their impact as fans of the brand is not lost on the Barely Famous stars.

"We do have a circle of friends that are very deeply enmeshed in the fashion world and when they love our clothes, it does mean a lot," Erin, 39, tells PEOPLE at the store opening. "It's a stamp of approval from girls who you really respect."

Sara, 40, echos the same sentiment saying, "We run in a circle where they buy high-end clothes, they wear designer stuff, so when they wear our stuff or like it, it does mean a lot. It's true. No matter how loyal they are, they are not going to be caught dead in something that they don't like."

But for Sara, there's something really special in seeing not-so-famous fans in their designs.

"Celebrities move the needle, but to me, the thing that feels the best is being out on the street and seeing a total random girl wearing Favorite Daughter. It feels amazing. I don't think you could ever get used to that," says Sara. "Our friends have to wear it. If they don't wear it, they're rude. So when I'm out in the world, and a random girl is like, 'Oh my god, I love Favorite Daughter' or she's wearing it, I die."

Adds Erin, "That's what we're doing this for, so that people will really love Favorite Daughter."

It's been one year since the sisters (whose father is music legend David Foster) launched the brand online, and say opening their boutique is the right next step for their line — and customers. "It was important for us because we pride ourselves on the quality," says Sara. "We are selling $300 cashmere sweaters and you need to be able to touch them and feel them to trust it."

"I think that we've established ourselves as a brand in [the past] year," Erin says of the company, which also hosted pop-ups throughout the country this past year. "We want to continue the momentum and not get quiet after that. And so the best way to do that I think is to have a presence in real life."

When designing the boutique, they created a space where they hope costumers feel inspired to relax. "I want them to come in here, sit on the couch, charge their phone and hang out. I want them to come in and take a picture against the striped wall. Content is king, you know, we wanted to create a space where people would want to take pics and tell their friends," says Sara.

The store was designed by model and influencer Fai Khadra, who was given the go-ahead to make the store "feel very alive and like it had character," says Erin. "He just had such a creative, cool vision, we just let him go with it."

Overall, they say they've grown this past year by making "a lot of mistakes" and learning how to "have one streamlined voice" even when they disagree.

Even though Erin says she wants "to kill [Sara] 7,000 times a day," they have established a great working-sister relationship.

"I will say that now that we have a team around us of people, we have a staff of people, we are part of a larger whole, we can't scream at each other in the middle of the day, we can't disagree, we have to find common ground," Erin adds. "So we've learned how to agree to disagree and allow the other person to make something that we don't believe in, but we know that they believe in it and like have to trust each other a little bit more."

Shop Favorite Daughter's new arrivals at shopfavoritedaughter.com.