Whether you read Laura's binge on the goblin fruit as a sexual escapade or as the beginning of a descent into chemical addiction and withdrawal, the poem certainly seems to want us to read it as some kind of a sin or transgression. Lizzie is able to resist temptation, but Laura and Jeanie (the girl who died) give in to their curiosity and desire for that tasty, tasty fruit. Many critics and readers think of Laura's temptation to eat the goblin fruit as an allusion to the temptation of Eve in the Garden of Eden in the Judeo-Christian creation story. But instead of getting kicked out of the Garden, like Eve did, Laura gets a second chance because of her sister.
Laura is a revolutionary figure in literary history: unlike most "fallen women" in Victorian fiction, Laura is neither killed off nor thrust into a convent, but is fully redeemed and allowed to complete the classic marriage plot.
Although Lizzie is the one who redeems Laura, it is Laura who gets the last word: uttering the "moral" of the poem fully reinstates Laura into normative Victorian morality.