Why does my husband fantasize about watching me and my twin sis together ually..??

Is this something to be worried about? Recently during our love making he has been telling me he would love to watch me and my twin sister fool around.. She could spend the night, blah blah.. I think what makes me gag is the fact that she is my sister,, ewww.. no way.. then he says what about your friend april... idk, what do u think??