I had been looking through many of the SVT covers posted here, and I kept staring at the Wakefield twins, because I realized that I knew the model. Then I remembered that I went to high school with a girl named Kim Matulova, who was a couple of grades under me - and that she was the model. I'm proud of her! She's been modeling since middle school; we all knew she would do extremely well. So now I'm hoping to get my hands on the books that feature her on the cover.
These are some images I found via Google that most closely matches how she looks on the covers:
It's been a few years, but I'm glad I recognized her. She was always very good at "fluid" poses, like the ones for the Sweet Valley books.