Each of the Argentium siblings can play at least two instruments, but the general rule is that one has to be ingrained in high society (piano, violin, flute, etc.) and the other relates to another culture (steel pan, zither, accordian, etc.). It supposedly adds to their whole "cultured" appearance that stems from their perfectionist mentality.

Wolfgang thinks it's unnecessary, since it adds on to their already bountiful amounts of self-induced stress.