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"We've been wanting to work together since you were born," Kimberly, 49, tells PEOPLE of her younger sister.
"Maybe the week I was born," adds Ashley, 42.
The two actresses are finally making their onscreen dreams come true via two Hallmark Channel Christmas movies, Sister Swap: A Hometown Holiday and Sister Swap: Christmas in the City, coming to the network in December.
"This specific project was Ashley's idea," says Kimberly. "We all went in with Ashley's husband Neal [Dodson], and we pitched it to Hallmark about five years ago. It took this long to figure out how we're going to execute it."
Kimberly and Ashley play sisters and appear together in both movies, which have overlapping elements. They also both serve as executive producers along with Neal.
"We used to do shows growing up, but I was always the director, producer, star," says Kimberly.
"She was very bossy," jokes Ashley.
"Incredibly bossy," Kimberly confirms. "And then when you got old enough, you finally started saying, 'No, I want to be in charge.'"
"Right, and that was really what worked for us," Ashley quips. "Once I got bossy, then we started making progress."
Jokes aside, "we had so much fun," says Kimberly. "It was really like one of the best experiences I've ever had."
"We had the best time," adds Ashley. "It was really fun, because we both got to work together for a long period. A lot of times the Hallmark movies you're in and out in three weeks, and this was a six-week shoot, which was really cool."
Each leads their own movie: Kimberly's is Sister Swap: A Hometown Holiday and airs on Dec. 5, and Ashley's is Sister Swap: Christmas in the City, which airs the following week, Dec. 12.
"I play Jennifer, she plays Meg, we play sisters [in both] movies," says Kimberly.
"The plots weave in and out of each other," Ashley explains. "And it's a lot like the way that Kim and I function in our relationship, which is we both are moms and we're businesswomen, but we're also best friends. And so our lives intersect a lot. It was fun to be able to weave that into Meg and Jennifer's storyline."
"We end up switching places," Ashley continues. "Essentially I take her job and she goes back to the hometown where I normally live. So we get to experience each other's worlds and there's a little bit of romantic connection in each of our cities. And then we get to come back together and hang out."
For more on the Williams sisters and the holiday TV movies premiering this year, pick up the latest issue of PEOPLE, on newsstands Friday, or subscribe here.
Both sisters appreciated the fact that they got to incorporate aspects of their real-life relationship into each film, and that the focus is on family.
"A lot of the time Hallmark movies I think are really fun to watch because it's about a guy and a girl and whether or not they're going to be together," says Ashley. "For the two of us, these movies were really about women supporting women and that was a really fun thing. That's a really important part of our relationship."
"Yes, about sisterhood," says Kimberly. "We rely on each other so much, and that's what these characters do as well."
The whole family affair extended to Kimberly's husband, country star Brad Paisley.
"My husband wrote a song for one of the movies. It's in my movie, and it's such a good song," she says. "So people should look out for that. That's going to be amazing."
Meanwhile, Kimberly and Ashley are thinking about how and when they're going to watch each film — possibly together as a big group and part of their family's Christmas movie-watching tradition.
"We could [watch them] on Christmas Eve? We usually do a viewing of a movie," says Ashley. "The question is, whose movie are we going to watch? I think we do a double feature."
"It's going to have to be a four-hour situation," notes Kimberly.
"Yeah, we can't play favorites," says Ashley. "We'll have to take breaks. A couple of bathroom and snack breaks!"
Sister Swap: A Hometown Holiday and Sister Swap: Christmas in the City premiere Dec. 5 and Dec. 12 on the Hallmark Channel.