The symbol of the femme fatale
Sirena's distinguishing characteristic is that when she discovers that her sisters have made her into a femme fatale, she decides to go be alone (perhaps forever), if that's the only way she can prevent herself from hurting men. However, her moral progress is far from over. She then gets a gift from the gods when she gets herself a heroic, young boyfriend. Then she must learn what it means not to prey on him in the relationship, and what it means to let him keep his autonomy, even if his destiny is to die in a meaningless war.
The motif of loneliness
Sirena is lonely three times, once before she found herself a boyfriend, once when she realized that relationship require a tremendous amount of work and sacrifice, and once when she is left at the shore when her boyfriend goes off to live his life without her.
The allegory of abuse
One way to understand the story of the sirens is that they represent an allegory against female mistreatment of men (perhaps misogynistically). However, bias or no bias, there is certainly a lesson to be learned: Adolescent traps trying to catch mates against their will are not honorable or healthy. That's one easy interpretation of the first act. Love is not love that must possess. That's another difficult lesson, and in all interpretations, the question will be learning to be independent, and learning to love others selflessly.
The troubling fate of the gods
When the gods act, there is always a sinister element to it, such that the characters in Greek literature often find themselves asking, "Why would they do this to me?" No doubt, Sirena finds herself in that place when the gods call the young hero back to his fate on the battle field.
The snake-bit lover
In Sirena's desperate attempts to be good and to resist her urge to seize control of her lover, she finds a lover in the person of Philoctetes, a man stranded by his fellow soldiers and left for dead after a snake bite. This snake bite might represent Philoctetes vulnerability to the insidious influence of a "Femme Fatale," and the two fall quickly in love.
