Dorothy Wordsworth is one of several remarkable women of the Romantic period (Mary Lamb is another) who are famous by association with their brothers. In our sexually egalitarian age, it is inevitable that their contributions to this fertile period of English writing should be reassessed. This, in part, is what Frances Wilson aims to do.

The book suggests that at least some of Wordsworth's poetic passages were borrowed, or lifted, from his sister's Grasmere journals. I remember at school doing a practical criticism on two comparable passages from the siblings' works and feeling, as Wilson does, that, where there are parallels, Dorothy's prose was more vital than her brother's verse. But these instances are few and one would have to award the creative laurels to William.

More originally, Wilson attempts to evaluate the psychological nature of the brother/sister dyad. She begins with an imaginary account of Dorothy's inward reactions to William's marriage to Mary Hutchinson. This is the book's one false note. From the sober tone of the rest of it, I suspect Wilson was encouraged into this doubtful practice by an editor who felt that the reader required something more than Dorothy's own extraordinary journal entry, which requires no false adornment.

After a night spent in a trance at Gallow Hill, the Yorkshire farm from which her beloved brother was to be married, Dorothy - who did not attend the wedding service - writes: “I gave him the wedding ring with how deep a blessing! I took it from my forefinger where I had worn it the whole of the night before - he slipped it again on to my finger and blessed me fervently.”

The ring, still extant, bears a Brussels mark, and it has been conjectured that Wordsworth bought it in Calais after a meeting with his French mistress, Annette Vallon, by whom he had a child, and whom he visited with his sister not long before his marriage.

Dorothy may not have attended her brother's wedding but she did accompany the couple, with no apparent rancour on their part, on their honeymoon. Moreover, try as they will, no one has successfully uncovered any harsh feelings between Dorothy and her sister-in-law, who appears to have been a woman of easy equanimity, and the marriage, a sexually full-blooded one, was counted uncommonly happy. Wilson's theory that the Wordsworths' undeniably close relationship prospered in a threesome is a much more compelling one than the tired old supposition that this was essentially sibling incest.

Salley Vickers's Where Three Roads Meet is published by Canongate

The Ballad of Dorothy Wordsworth by Frances Wilson
Faber, £18.99

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