The women co-starred in 'The Parent Trap' in 1998.
"We talk through social media mostly," Walter told ET of herself and Lohan. "I sent something through her dad to her for the baby, for the new mom. Usually it's just commenting on each other's posts and me being maternal to her when nobody asked me to be and giving her advice."
When Lohan announced that she and her husband, Bader Shammas, were expecting a baby in March, Walter said she "was just thrilled that she was going to have this little one."
"I think it's exactly the right time in her life," Walter said of Lohan, who starred in The Parent Trap as twins Hallie and Annie. "She's clearly in love with her handsome husband, and she was always going to be a good mom."
Walter has long been sure of Lohan's maternal instincts, since she watched the then-11-year-old actress with her siblings on the set of The Parent Trap, which came out in 1998.
"I remember her on set, when we were shooting the movie, she would walk around with the littlest one, who was two at the time, Dakota, and she would plop that baby on her hip and just march around with him and take care of him," Walter recalled of Lohan's now 27-year-old brother. "They're really close, the sibs, and she's, the oldest, so she was always very maternal, even then."
Walter, who played Chessy in The Parent Trap, added, "I think she's gonna be a great mom. I'm really happy for her."
ET's interview with Walter came ahead of the actress' appearance on Ancestry's new YouTube series, unFamiliar. On her episode of the show, which takes celebrities and their relatives into untold stories of their past, Walter learns that she and her best friend, Elaine Hendrix, The Parent Trap's Meredith Blake, have a century-old connection.
"For us both to wind up in Los Angeles being like, 'Look at me!' maybe some of it is fate, but to me it's just the idea of our individual destiny, my soul and her soul were meant to be connected," Walter said of the discovery that her and Hendrix's great-grandparents lived steps away from each other in New York City in the early 1900s. "We're more than best friends. People say 'Oh, that's my bestie.' I mean, we're like soulmates."