A sister joins the Brotherhood Christina Rossetti is admitted to the Pre-Raphaelite club. Iain Gale applauds

On 12 April 1853, four young men sat in a small London studio drawing each others' portrait. The Pre-Raphaelite Brotherhood was holding one of its last meetings. Here were all the original members: the painters Dante Gabriel Rossetti, William Holm an Hunt and John Everett Millais and Rossetti's brother William Michael, a writer. But someone was missing. Christina Rossetti, poet and Pre-Raphaelite in all but name, excluded by virtue of her sex, is now the subject of an exhibition which places her w ithinthe circle of the Brotherhood.

Christina was a natural Pre-Raphaelite. Her ethereal, slightly mournful face, seen here across the years in 14 portraits by her brother, was the first of a now familiar archetype. She provided the model for Rossetti's early paintings, including The Girlhood of Mary the Virgin, the first exhibited with the cryptic inscription PRB, also on view here. But her involvement was more than that of a mere muse.

From the outset, Pre-Raphaelitism was as much about poetry as it was about painting. Both satisfied the brotherhood's creed: "to have genuine ideas to express . . . to sympathize with what is direct and serious and heartfelt in previous art, to the exclusion of what is conventional and self-parading and learned by rote." Throughout his career, Dante, the movement's driving force, wrote poems to accompany his paintings and illustrated the poetry of others: Keats, Tennyson, Patmore and, not least, his sis ter. The ease with which he complements her Goblin Market and The Prince's Progress, bears witness to the proximity of their intentions.

It was taken as read that Dante's talented sister would contribute her verses to the Pre-Raphaelites' short-lived house magazine The Germ and it is significant that the first of these, "Dreamland"and "An End", both deal with early death, a recurrent theme in Pre-Raphaelite painting. Ever y one of the Pre-Raphaelites' limited range of subjects: Christian doctrine, medieval myth, the morality of contemporary life and scenes from literature recurs frequently throughout Christina's oeuvre.

On the simplest level, her poems and prose echo Pre-Raphaelite symbolist iconography. The courtly love affairs, princes and monsters of her brother's watercolours are present in abundance. Here too are the strayed sheep and fallen women of Holman Hunt and Millais' dreamy-eyed children and tragic lovers. Christina's similar treatment of the same themes, however, derives from a very different source to that of the painters whose faces stare from the walls of these galleries. While her brother's art was fired by the volatile combination of vivid imagination, self-pity and romantic role-playing, the realist strain of melancholy found in such poems by Christina as "Remember" was drawn from an understanding of real life. Her breakdown at the age of 14 was followed by two blighted engagements and other unconsummated attachments. There is also the possibility, often hypothesised, of early sexual abuse.

Christina's best-known poem, Goblin Market, is suffused with a disturbing eroticism close to that discernible in the mature paintings of her brother, their sexuality barely concealed beneath myth and metaphor.

Dante's decline from the lion-haired youth of his early self-portrait to the puffy sot captured here in Lewis Carroll's 1863 photograph is well known. Less so is his ambivalent attitude towards women. While encouraging the artistic endeavours of his beloved Lizzie Siddal, he was moving ever closer to the awe-struck isolation of his later works, with their portrayal of woman as a knowing enchantress. While remarking on this in her poetry, Christina herself was guilty of a similar duality towards woman's position in society - half embracing feminism, half denying it. In her 1856 poem "The Triad", she portrays the lives of three women: the wronged wife, the mistress and the spinster, as equally wasted on the love of one man. It was this trio, though oftendepicted from a contrary standpoint, that provided Pre-Raphaelitism's most enduring images.

As Pre-Raphaelite angels throng our Christmas cards and Christina's words to "In the Bleak Midwinter" receive their annual airing, here is a reminder of the movement's real importance. With their manifesto and their radical doctrine, the Pre-Raphaelites were the first self-consciously avant-garde art rebels. Christina shared their aspirations and contributed to their achievement. Hopefully this fascinating exhibition and a scholarly new biography, will restore her to the status she deserves as the true Pre-Raphaelite Sister.

National Portrait Gallery, London (071-306 0055), To 28 Jan `Christina Rossetti' by Jan Marsh (Jonathan Cape, £25)