Alternately, one could assume that the breastbone was incorporated into the harp as something like an inset decorative panel rather than a structural member.

Mind you, to modern ears, the idea of somebody mutilating an unknown body for harp bits is more jarring than mere sororicide, which is just tabloid material.

But yes, depending on the theme of your story, I'd probably myself just use the hair. I can see a mindset in which one might use a dead woman's hair as a form of tribute or protest, while hacking up her corpse isn't quite the same.