In what way do the swans in “the wild swans at coole” by william butler yeats represent eternity?

Their hearts have not grown old; 
Passion or conquest, wander where they will, 
Attend upon them still. I would say that this section of the poem indicate that the swans represent eternity ie saying that their hearts have not grown old and that they still have passion and a will for conquest indicates at least for now eternal qualities that they possess,

What sisters? is it in a story?