Now I'm really confused. I thought it should be "Ce sont des artistes" because you have to say "C'est mon père" (not "Il est mon père") for "He is my father" and "Ce sont des musiciens" (not "Ils sont des musiciens") for "They are musicians." "Il est" and so on are correct if followed by an adjective: "Elle est géniale," for example. All wrong?
It is because "artiste" is considered to be an occupation here. You are sort of on the right track. (Though you could say Ils sont musiciens without the des).
See the following - Remy explains it well : https://www.duolingo.com/comment/4021012
Also, as des is the plural of un/une, you can extrapolate the plural from the explanation given by Académie française here: http://www.academie-francaise.fr/il-est-cest-un-0
Bonne chance !