Her ribs are called costae, they do not connect in the middle and can be flared wider, or partially interlaced to squeeze through thinner holes. Her lung also extends through most of her torso, and she only has one, very long one, like a snake. Unlike a snake, her heart locates in her human half of the body, but even there her ribs do not seal into a ribcage as it would on a human, so she is more bendy even on upper body than a human would be. her lungs also begin from just below her human hips, which are actually just larger muscles and thicker ribs.
Her voicebox is up in her thorax, further from the lungs, this limits her maximum voice volume from average human, and makes her voice also have a higher pitch than some humans to make most of the air that can flow out. While she lacks volume, she is a great songstress as her lung capacity is insanely large. Making her well suited for more slow mellow ballads that hold notes a long time. Also brighter voices carry further than low ones, which compensates for lack of volume.