Things, and even sisters, are not what they seem when Hayley Mills plays identical twins in The Disney Channel's sequel to "The Parent Trap." In these scenes from "Parent Trap II," airing this summer, Hayley jokingly wears a brunette wig to thicken the comedy's plot.

Hayley Mills was a phenomenon of the 1960s, a teenage superstar with six Disney hits ("Pollyanna" 1960; "The Parent Trap" 1961; "In Search of the Castaways" 1962; "Summer Magic" 1963; "The Moon-Spinners" 1964; "That Darn Cat" 1965) and legions of fans who sent her 7,500 letters weekly. Today that avalanche has subsided but as the post office at The Walt Disney Company testifies, no week has passed in 26 years without its packet of fan mail for Hayley. In July, 1983, two-and-a-half months after its launch, The Disney Channel aired "Pollyanna" and Hayley's comet blazed again. Subscribers clamored for more, the Channel obliged, the fan mail swelled, and it became clear that a contemporary comedy starring Hayley Mills was imperative. And as a result, "Parent Trap II" premieres on The Channel this summer.

"'The Parent Trap' was my favorite film," Hayley said recently, "so I welcomed the chance to work for Disney again. I wasn't sure, though, that after 25 years I could do justice to the original. But everyone at Disney was so enthusiastic about the project that I finally said yes. And I think it's turned out well."

This time Hayley reprises her dual role as Sharon and Susan, identical twins who now are adults. But instead of plotting to reunite their divorced parents, as they did in the first movie, they find themselves enmeshed in a matchmaking scheme woven by Sharon's daughter.

"The twins have grown up quite nicely, I think" Hayley said. 'Twenty-five years is a big gap and an awful lot happens to a person, but the girls are basically the same. We all stay the same inside. I think they're quite fun."

Certainly a lot has happened to Hayley since she left the Disney organization at age 19.

"I was becoming a young woman during the '60s, and that was a pretty wild time for me," she said. "I'd been sheltered by the studio and over- protected by my parents (actor John Mills and playwright Mary Hayley Bell). I had a protracted adolescence, I lost self-confidence, and in fact I even lost my identity for a while. As an actress I projected a kind of impulsive, spontaneous, happy-go-lucky personality, although as a young woman I was becoming much more complex. I simply didn't know who I really was, but I did keep on working. It was like my lifebelt."

At age 20 she returned to her home in England and began seeking "interesting, more mature roles. But because the Disney films were so popular," she said, "it took forever and a day to shake off that squeaky-clean image." After making a number of so-so British pictures, Hayley decided to give up acting for good. "I stopped working for a year," she said. "Then I re-thought the whole thing and admitted that I did not want to quit forever." She turned her back on films, though, and launched a new career in television and the theater, where she has become eminently successful.

The Hayley Mills of today is a radiant, blue-eyed blonde of 40 with two sons of her own, Crispian Boulting, 13, and Jason Lawson, 9. Crispian, she said, wants desperately to be an actor. "He's been offered a job in a television series by the BBC. He's also taking his entrance exams to go to a public school in England. At the moment I think that's more important. But I'm not surprised that he's interested in acting. I am, his father (the late director Roy Boulting) was, and so is his grandfather. It's in his blood on both sides. Jason, now, takes everything very much in stride. He loves coming to the studio or theater when I'm working, and hanging about, chatting with everybody. But he hasn't shown any particular interest in acting. At this point he thinks he'd like to own a pub or drive a taxi."

Hayley and the boys live in a house outside Hampton Court near London, a half-hour's drive from her parents' home there. "We're a close family," she said. "I see my parents frequently, and I try to visit my sister Juliet in California every year.

"I'd love to do more work over here, but my home is in England and, unless you're right under their noses, people tend to forget you. Indeed, they wonder if you're dead or if you've retired and are out growing mushrooms somewhere. They have to be reminded that you're still at it."

Hayley's fondness for the Disney organization has remained steadfast through the years. In 1981 she returned to host "Disney Animation: The Illusion of Life," a behind-the-scenes TV tour of the studio. In 1983 she narrated "When You Wish Upon a Star," a monumental BBC radio series consisting of ten 90-minute programs documenting Wait Disney's career and achievements. This summer she plans to be back in the States with her children. "And of course we're looking forward to visiting Disneyland and Wait Disney World," she said. "The boys are really excited because they haven't been to Florida. That will be a big adventure for them."