The Ballad of Dorothy Wordsworth, by Frances Wilson

The true story of the mouse and the turtle dove

Whenever we think of Dorothy Wordsworth, we are aware of two things of which her contemporaries were largely ignorant: the unsisterly fervour of her love for her brother William, and the extent of the debt his poetry owed to her. These aspects of their relationship emerged over years, as editors of her journals became less concerned with maintaining the comforting image of Dorothy as devoted sister and less fearful of engaging with the realities of her relationship with William, and her literary importance in the Romantic canon.

In this intelligent and well-written biography, Frances Wilson unpicks Dorothy's "Grasmere" journals – which suggested so much of the material her brother worked into his poetry – for clues towards a clearer definition of their passionate attachment. Siblings who grow up apart and become reacquainted as adults are known to be more prone to "inappropriate" feelings towards each other than those who develop under the same roof. On the death of their mother in 1778, Dorothy and her four brothers were separated and did not spend any time together until, aged 15 and 16, Dorothy and William were partially reunited.

They quickly established their ideal – to share a home – which they did six years later. They lived together for the rest of their lives and there was no question of Dorothy moving out when her brother married Mary Hutchinson. A woman of unique qualities, Mary sublimated her own needs as a wife more successfully than Dorothy sublimated her own as William's no longer exclusive partner.

Wilson poses many questions about the nature of William and Dorothy's intimacy, some of which could doubtless be answered by the restoration of lost journals, missing pages and excised lines. One famous passage which has been deciphered records how, the night before William's marriage, Dorothy slept with the wedding ring on her finger, returning it in the morning. The image remains troubling yet inscrutable: is this a divorce before a marriage or does it mark the sealing of a trinity? Is there a sexual aspect and, if so, what is it? The more one learns about Dorothy and her "Beloved", the less one seems to know.

Wilson sees symbols everywhere in Dorothy's descriptive writing, in the solitary flower, in the relationship between the moon and the stars, in the landscape and the light upon it. She also wonders how much the mind supplied what Dorothy saw, suggesting that some of the peculiar qualities of light Dorothy reported could have been due to symptoms of migraine.

We don't know what caused Dorothy's many headaches. It could have been the stress of repressing her feelings for her brother – or repressing her feelings for others in order to preserve the unassailability of the bond between William and herself.

Her headaches could have been related to the mental illness that blighted the latter part of her life, during which she became infantile, tyrannical, violent, incoherent and terrifying to strangers. They could have just been headaches, or merely an excuse to escape what she could not express.

Dorothy had a soft spot for true stories and collected the tales of the vagrants who passed her door with the same interest with which she gathered images from nature. One of the stories she recorded, that of Barbara Wilson's Turtle Dove, she wrote down at her brother's request, shortly before his marriage. "She had 2 turtle doves. One of them died the first year I think. The other bird continued to live alone in its cage for 9 years, but for one whole year it had a companion & daily visitor, a little mouse that used to come & feed with it, & the Dove would caress it, & cower over it with its wings, & make a loving noise to it. The mouse though it did not testify equal delight in the Dove's company yet it was at perfect ease. The poor mouse disappeared & the Dove was left solitary till its Death."

For Wilson, this anecdote is pregnant with meaning, but we have no way of knowing why Dorothy and William were drawn to it. What we do know is that the mouse was an inappropriate object of the dove's affection and that, however it ended, it couldn't last.

Sarah Burton's biography of Charles and Mary Lamb, 'A Double Life', is published by Penguin