'The first instance of a perfectly painted twilight...'

It's high time we had a painting - and what painting could be more perfectly seasonal than Millais's melancholy masterpiece Autumn Leaves? It's a picture I've lived with quite a bit over the years: there was a print of it in my rooms at university, and we have a (less satisfactory) reproduction of it in the front parlour of our present house. It's an endlessly fascinating work.
 The painting dates from 1856, when Millais's great Pre-Raphaelite works were behind him and his long descent into facile whimsy some way ahead. He was still young (27), professionally successful, happily married to Effie Gray - formerly the desperately unhappy Mrs Ruskin - and they were starting what was to be a numerous family. And yet Autumn Leaves, whatever else it is, is a deeply sad painting, suffused with a sense of transience and loss and, yes, death.
 According to Effie, Millais's aim was to paint a picture that was 'full of beauty and without a subject' - a kind of tone poem, you could say. The artist also 'intended the picture to awaken by its solemnity the deepest religious reflection. I chose the subject of burning leaves as most calculated to produce this feeling.' He also sought to evoke a Tennysonian mood - Tennyson being a master of the elegiac tone, and Millais his greatest pictorial interpreter.  The picture even drew on Millais's memory of raking leaves with Tennyson at the poet's house (somehow one suspects that Millais was the more efficient raker).
 Autumn Leaves was a hit with the critics, not least Ruskin (whose disastrous marriage of Effie had been annulled only the previous year). It was, Ruskin declared, 'by much the most poetical work the painter has yet conceived; and also, as far as I know, the first instance of a perfectly painted twilight'. (It is also, he might have added, a perfect representation of the full glorious tonal range of autumn leaves.) Ruskin even invoked the name of one of his artistic gods - 'though Giorgione might have come nearer the glow, he never gave the valley mist. Note also the subtle difference between the purple of the long nearer range of hills and the blue of the distant peak.' Indeed.
 Who are these girls raking leaves? We know that the two figures on the left are sisters-in-law of Millais - Alice Grey and, staring full at us, Sophy, the troubled teenage girl with whom the artist had an intense emotional relationship, and who later slipped into an unhappy condition we would now label as Anorexia Nervosa, dying young. The little girl on the right, holding the (meaningful) apple, is a Miss Smythe of Methven. But who is the girl who is standing, rake in hand, eyes downcast, behind and apart from the main group? This figure seems to be in a different register - not only pictorially - from the others, almost to be inhabiting a different space. There is no interaction between her and the other girls, or between her and the artist/us; she looks inward and stands apart. Indeed the painting would still make perfect pictorial sense if she was subtracted from it. I suspect that she is indeed not physically present in the scene, that she represents someone recently dead - perhaps another sister of Effie's? I haven't been able to find any documentary evidence for this, but is seems to me quite likely (and very 'Victorian'). Has anyone any information? I'd love to know...