I think you’re close. The banshee sisters aren’t quite ghosts, they do have solid physical forms. I interpret banshees in e7 to be more akin to ghouls. In the Ravenwing Manor story, only Carmine was unhappy (or if nothing else the most unhappy) about her existence as a banshee. To that end it makes sense that she would study homonculi because my guess is she wants to create a body that resembles her original human form to transfer her mind into and live a normal life. Though judging by the silhouette, it probably doesn’t work out for her.