"There's a significant age gap which feels almost nonexistent because she's now a mom," Robyn said. "We have so many similarities. Our relationship is on a deeper level. We used to help each other with our projects and lines, things like that. Now we're doing the mom thing together."
"I genuinely feel like her children are mine as if I gave birth to them, and vice versa," she added with a laugh. "We're like, 'Your children are mine too.' Our relationship has only gotten richer and fuller."
Robyn and Blake are half-sisters and have the same mother, Elaine Lively. Robyn shares three children with husband Bart Johnson: Baylen, 19, Kate, 17, and Wyatt, 14. Like Robyn, Blake also has three kids that she shares with husband Ryan Reynolds: James, 7, Inez, 5, and Betty, 2.
Since they are "so in tune" with one another and the other's children, the "Doogie Howser, M.D." star said she and Blake don't feel the need to give each other advice about motherhood.
"I've learned so much from her, and she’s learned so much from me," Robyn told Fox News Digital. "I never go in and say, 'Hey, you should really do this.' I think we're constantly observing and learning from each other. I see things that she does with her kids, and I’m like, 'Oh, that is so great. That’s so wonderful and special.' Or I'll go, 'I love how she handled that' and then implement that in my life with my kids. And she does the same. It's like you don't have to have those types of conversations because you're so in tune [as sisters]. And we've had so many shared experiences together."
Speaking about his wife and sister-in-law's relationship, Johnson, 51, said, "I'll say from my perspective, they're like the same person. They've got the same energy and the same sense of humor. They're also so connected and love each other so much."
"It's really gone a long way since Robyn was on 'Savannah' and Blake would come to hang out on set as a little girl," he added, referencing the 1996 soap opera. "Now it feels like that age gap has closed… Our kids are the center of our entire universe. [Robyn and Blake] share the love and the passion for motherhood. They’re best friends. It’s such a charming friendship. It's a beautiful thing."
Meanwhile, also during the interview, Robyn and Johnson spoke about their career, including what it was like working together in their new film, "Strong Fathers, Strong Daughters," in which they play a husband and wife on-screen.
The couple -- who will celebrate their 23rd wedding anniversary next month -- also discussed why they choose certain projects, with Robyn explaining the significance of being involved with family-friend projects. ("Strong Fathers, Strong Daughters" is available to stream on Pure Flix, which produces faith and family entertainment.)
"I see shows now, and I feel like Hollywood is trying so hard to push the envelope and I just don't think it's necessary," she told Fox News Digital. "I think we're just oversaturated with stuff like that. So to be part of a project where you can share it with the whole family and offer a great message, it's so important. And I think people are craving for that. I'm super proud to be part of a film like this one. It’s sweet, it’s funny, and I can tell anybody to watch this. I'm so confident of it."
"It does feel like the priority has been to push the envelope and how dark we can go," she added. "That's what everybody is doing now. So this feels pretty unique in that we can just focus on the story and the relationships between family members, which has plenty of drama and conflict -- relatable conflict for us. It's really enjoyable to just not have any sort of agenda other than telling a great story."