Which ran in the veins of my fathers and ours,
When we were in our youth and had one heart,
And loved each other as we should not love,
And this was shed."
In a poem published a year later, The Duel, there is also a reference to blood—"And then there was the curse of blood." This line and the passage in Manfred merely refer to the fact that an ancestor of Byron, the fifth Lord, not a direct ancestor, killed Mr. Chaworth whose blood flowed in Mary's veins. Astarte then in Manfred is Mary Chaworth and not Augusta.
Shelley had a great affection for his sister Elizabeth and wanted his friend Hogg to marry her. She returned to her father and Shelley was broken hearted that she drifted away from his own influence. He thought she was not lost to him and wanted to take her with him to the west of Ireland in 1814. He continued to love her, and this influenced his work. In the first edition of the Revolt of Islam he made Laon and Cynthia, who were brother and sister, lovers. The publisher made the poet regretfully change certain passages, mostly single lines. In the early preface the poet concluded he could not see why an innocent act like love of brother and sister for each other should arouse the hatred of the multitude.
In Rosalind and Helen he describes Helen visiting a spot where a sister and brother had given themselves up to one another, and had a child, who was torn by people limb from limb. The mother was stabbed while the youth was saved by a priest, to be burned for God's grace. Their ghosts visited the spot.