Sidelight windows might seem challenging to cover, but Interiors by Peggy @ The Grand Design offers a variety of window treatments that come in narrow widths:
Duette® Honeycomb Shades, are offered in almost any shape and size. Plus, they have the added benefit of a cellular construction that creates an extra layer of insulation at the window.
Silhouette® Window Shadings look great on small windows. They have S-shaped vanes that appear to be floating between two sheers. The sheers draw sunlight into the room, while the vanes tilt to control light.
Luminette® Privacy Sheers are a vertical window treatment that looks great on sidelights, featuring a sheer fabric face to let light in and soft fabric vanes to keep light out.
Vignette Modern Roman Shades are another window treatment option that's available in narrow widths.
Scroll down for more pictures.