The term 'Glockenspiel Cousin' is referring to the musical instrument known as the Celesta. The Celesta is a keyboard instrument that produces a sound similar to a Glockenspiel, but with a softer and more mellow tone. It is made up of metal bars that are struck by small hammers connected to a keyboard. The Celesta was invented in 1886 by Auguste Mustel and has been used in many classical compositions since then.