"We always want to make sure that we are wearing things that represent our personality and that we can live comfortably in," the sisters tell PEOPLE exclusively of their debut clothing collection Erin Foster and Sara Foster are keeping it real when it comes to working together as siblings.
Ahead of the release of their new clothing line, Favorite Daughter — which launches today — the two sisters spoke to PEOPLE about the collection and the challenges that come with collaborating with family.
"Erin's very opinionated and I'm very opinionated, but I think we just keep the focus being on what our North Star is, and that is success in what we're doing," Sara, 39, says.
"But, it's not easy," she continues. "In a normal work setting, you really have to hold your tone. You're not allowed to say exactly what's on your mind or you might really offend somebody. And when it's siblings, that doesn't necessarily apply."
Sara adds: "But then, at the same time, because we are so honest with each other, we've been able to really get things done and I think we're really proud of the ventures that we are in together."
However, Erin, 38, feels a bit differently and has a more cynical take on working alongside her sister. "It's awful," she jokes.
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The line kicks off with a holiday collection featuring an array of stylish and comfortable clothing pieces — including faux leather leggings and curve-hugging skirts, turtleneck sweaters and satin bodysuits, and the sisters' favorite item, a hoodie with a built-in face mask.
Matching style with comfort, the two say, was a very important part of the clothing line for them. "It's literally the backbone of everything we wanted to do," Sara notes, as Erin adds that the duo "are not really people who are willing to suffer for fashion."
"We always want to make sure that we are wearing things that represent our personality and that we can live comfortably in," Erin continues. "And we always want to meet at the intersection of appropriate pricing, comfortable and fashionable."
Sara adds: "It was imperative to us that we were able to source materials that would feel amazing on your body and would be easy to put on, but still look incredibly stylish."
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They teamed up with their other sister, stylist Jordan Foster, when creating the line, but Sara makes sure to clarify, they don't consider themselves "fashion designers."
"We never saw ourselves as clothing designers," Sara says. "We have an amazing designer who is at the helm of the ship. But really the idea was to create and design things that we would want in our closet, but don't have."
She continues, "So we wanted to create something that was sort of for the girl that is kind of tired of shopping at Zara and wants sort of a level above that, but doesn't really want to break the bank, but still look stylish. So it's been fun to design things that I love that I want my closet, but Erin wouldn't necessarily wear and vice versa."
The two sisters, who starred together on their 2015 TV show, Barely Famous, didn't start working with one another "until a little bit later in life," as Sara notes, because "we were really sort of on our own path." Now, they say, their relationship has "evolved a lot because we rely on each other heavily for our work."
"We're working together, but we're also family," says Erin. "So it forces you to constantly face someone who knows all of your buttons and exactly where they are and how to push [them] ... It's a complicated dynamic, but it's also really unique one."
"If it wasn't for working together, we probably wouldn't be speaking because we're both so stubborn," Sara continues. "When we get into a fight, we could probably go like a month without calling each other. But because we have to speak every day for our businesses, we are forced to make up."
"Like, we'll scream and hang up on each other and then an hour later we'll be on a Zoom together and I'm like 'F--- my life. I have to talk to this bitch one more time?'" she adds.
RELATED VIDEO: Family Means Everything to the Fosters
Like it or not, family is ingrained in the brand and even inspired the name, Favorite Daughter, which Erin came up with when starting an LLC a few years ago.
She says she had the idea "because I figured everybody always thinks that they're the favorite."
"Our dad [David Foster] has five daughters and so he used to always kind of secretly say to one of us, 'Honestly, you're my favorite,'" she says.
"And moving into now starting our own clothing line, it just felt like the obvious name," Sara adds. "It was just ironic that it stemmed from something from so long ago."
Fans can shop the Favorite Daughter holiday collection (which ranges from $98 - $398) now at shopfavoritedaughter.com and Nordstrom stores.