I don’t think the habits are fictional in the movie.Maria Trapp I think was in the Benedictine Monastery.I have seen old photos of Benedictine nuns wearing the pleated wimple, like in the movie, but I have forgotten where I saw these photos.
Also, in the recent movie with Phillip Seymour Hoffman were he plays a priest and Meryl Streep as a nun, the habit is that of the Sisters of Charity, the ones founded by St.Elizabeth Ann Seton. And the habit was correct in the movie.
Since the Sound of Music was autobiographical, I think they tried to be correct in what the sisters wore. Maria von Trapp was alive and well at the time, as was the rest of the family, if they saw any of the filming of the picture, before it was released, i’m sure she would have made comments to the director and producer about the sisters habit if it was correct or not.