The Brotherhood soon drifted apart- for all sorts of reasons. Brotherhoods do. The only one of the original three to keep on painting in the style they'd worked out together was Hunt. But Hunt was a loner- and leadership of the continuing movement- insofar as it was a movement, insofar as it was continuing- fell onto the shoulders of the intensely charismatic Rossetti.
He had always been a reluctant realist. Where Millais and Hunt had happily painted scenes from "modern life", Rossetti attempted only one- the stilted morality picture "Found" and abandoned it half-finished. His heart was wholly in the middle-ages. It wasn't just the art he loved, it was the ethos- as filtered through the romanticism of Scott, Keats, Tennyson- and his most characteristic, early Pre-Raphaelite works are highly scrubbed, visionary water-colours of scenes from Dante and the Arthurian cycle. He gathered disciples (and lovers).- he was the sort of man who did- and these disciples- including men of considerable talent like William Morris and Edward Burne-Jones- created the school (very loose, anyone who was into damozels could join) of second-generation Pre-Raphaelitism. Where the first generation were all about style; the second generation were all about content. The first generation were realists, the second (and third and fourth) were mystics, symbolists, late romantics. With Morris- arguably the greatest of them all- medievalism turned political- and fed into guild socialism and the arts and crafts movement.
There is really nothing to connect the hard bright realism of early Millais with the swoony artifice of late Burne-Jones except the shadow of Rossetti. Pre-Raphaelitism is really two, quite distinct things: a short-lived, revolutionary art-movement of the 1850s- and an aesthetic mood that brooded over the second half of the 19th century and persisted well into the 20th. The fact that they've got muddled up together is an accident- an accident of leadership- in effect, an accident of personality.
Millais: Christ in the House of his Parents
Rossetti: The Wedding of St. George
Burne-Jones: The Garden Court