"Barbara," the younger sister, sure looks like Kim Cattrall! I looked up her bio and it says Kim was born in 1956, which would have made her 13 when this movie was produced. "Barbara," is obviously not 13, but both actresses have exactly the same nose, eye brows, and eye width! Check out the 1988 movie, "Masquerade," that Kim did with Rob Lowe.
I wonder if this is where "Sleepy Head" (1973 Georgina Spelvin Tina Russell) took it's inspiration from. There are some similarities in the situations and story arc.