Please summarize "Sister Helen" by Dante Gabriel Rossetti, including plot, main characters, and supernatural elements.
"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.