The plot of the ballad "Sister Helen" can be summarized as follows: Sister Helen is in a dialogue with Little Brother. Through this conversation, we learn that, in medieval Ireland, Sister Helen is killing her beloved, Keith of Ewern, who betrayed her by marrying someone else. She begins her revenge plot on the morning of his wedding with another woman by slowly—over three days—melting a wax figure she made of him.

The characters in the ballad include the angry and persistent Sister Helen, Little Brother, and Keith of Westholm, who comes to plead with Sister Helen on the dying Keith of Ewern's behalf. Westholm tells her Keith of Ewern wants to meet with her, but as far as Sister Helen is concerned, it's too late for that. Then Keith of Keith, Keith of Ewern's father, arrives to make a plea for his son's life, kneeling in the road. Keith of Ewern's new wife also shows up, golden haired and sobbing, hoping to save her husband. None of these characters can persuade Sister Helen to alter her decision.

The main supernatural element of the poem is Sister Helen's witchcraft. Melting the effigy does succeed in sickening and killing the real Keith of Ewern, so the witchcraft works.

 "Sister Helen" by Dante Gabriel Rossetti was published in 1853. It imitates the traditional ballad in its use of a refrain and dialogue.  

The main characters in the story are the eponymous "Sister Helen" and her brother, who are the speakers of the poem, and the Keith family.

The basic plot of the poem is simple. Keith of Ewern jilted Helen. She has decided to take revenge, killing him by magical means, by creating a wax effigy of him and melting the effigy in the fire over a period of three days, starting with the day of his marriage. Her brother (probably much younger than  herself) is watching the process and asking questions.

Four members of Keith's family come to ask Helen to have mercy on him as he sickens, his two bothers, his father, and his wife. She is unrelenting, and he dies. 

The parenthetical mentions of Mary refer to the Virgin Mary, commonly addressed in prayer by Roman Catholics, and the variations on the refrain suggest that each day, as Helen persists in her witchcraft, she is killing her immortal soul as well as her ex-lover's body.