Lyric discussion by Letmein 

Cover art for The Bonny Swans lyrics by Loreena McKennitt

While the older sister does murder the younger, she does not literally turn into a swan. Instead her body floats downstream sometimes floating, sometimes submerged to where the miller's daughter fishes her out. The harper then uses her bones to make a harp and strings it with her hair. This is pretty creepy but apparently he's doing it so her ghost can accuse her killer. Incidentally this is a classic fairy tale though little known these days. Too gruesome for the kiddies