Historically, chair rails were used to protect plaster when chairs were backed up against the walls to clear space in the middle of a room. Like many functional inventions, this one became more decorative over the years. It became common to use the rail as a divider strip for paint or wallpaper to create a contrasting look. Chair rails are still used today to create a more formal and traditional design. If that's the look your sister wants, chair rails would be a good choice. But other strategies would be better if she seeks a more contemporary feeling. Decorating Gallery is filled with good ideas.