The case of singing siblings Nigel and Christopher isn't unique, but it's damned rare. The soprano Kristine Ciesinski has a mezzo sister, Katherine; Terry and Neil Jenkins have been known to play Happy Families, but discounting the Everly Brothers and the Nolan Sisters, that's about it.
What is unique about the pair of them, aside from the unusual pairing of tenor and counter-tenor, is their acting talent. These two aren't just international soloists who sing on stage, they are genuine operatic animals. Cast either of them and you can wave good-bye to the old-fashioned "stand and deliver" performance style. Both are more than capable of producing honeyed tones, but these two give you something bolder, richer and altogether more theatrical. Some directors (and particularly record producers) favour evenness of sound above all else. It's a little reminiscent of the Tebaldi and Callas debate: purity versus passion. Luckily for anyone going to Glyndebourne's inspirational staging of Handel's oratorio Theodora, Peter Sellars has opted for the dramatic approach.
What he cannot have known is just how ideally suited these two are to playing the roles of the Roman commanding officer Septimius and his friend Didymus, a convert to the forbidden faith of Christianity. The Robsons' parents were officers in the Salvation Army, but they are both keen to dispel any notions of bible-bashing and Oranges Are Not the Only Fruit- style repression. Not only was theirs an enlightened evangelical environment, music was central to their lives. They sang constantly and played brass instruments in the Salvation Army band. "We even sang grace around the table in slightly improvised four-part harmony," says Nigel, the elder of the two, and the second of four sons. Their father was constantly writing music for the church but, again, not the imagined four-square English hymnal stuff. Nigel remembers that, as early as 1948, within three years of its premiere, his father had the sheet music for two arias from Britten's Peter Grimes that he wanted to sing.
"It was all part and parcel really. We were encouraged in music," observes Chris, the counter-tenor, "but there was never any pressure, just as there was no pressure to join the Salvation Army. There was no pressure on us to do anything other than what we chose to do."
Neither of them is a practising Christian any longer, but their father's influence is there for all to see. Both have reputations as 20th-century music specialists, traceable back to their monthly record allowance and their father's encouraging them to listen to Messiaen and Stockhausen. He was also responsible for their interest in performing. "Dad used to do these evangelical musicals," says Nigel, "cobbling together bits of operetta and so on. It feels a bit embarrassing looking back." ("Awful," laughs Chris.) "Things like Salvation Ship Ahoy!, a sort of Billy Budd for the Lord." ("Jesus saves / in the waves," giggles Chris.) "He also did this strange thing, Three Faces of Eve, which was bits of a play plus Vaughan Williams's music for Job, over which he narrated the story of creation while people would mime it in some way, like dance."
Yet, for all their mixed feelings about the "church operas", both brothers recognise the experience as having ignited very strong feelings in them about theatre. Professional singing, however, was not immediately on the agenda. Nigel went to York University as an organist and composer, while Chris went to Trinity Music College as a trumpet player.
"My voice came down very slowly over a year," he recalls. "When I went to Cambridge Tech, aged nearly 17, it had settled into a light tenor / baritone, but I didn't sing. Then, at the beginning of my second year, someone heard me mucking about in the practice room and said I should have lessons, so I started singing tenor. One day I went straight from a trumpet lesson to a singing lesson, which was unusual. When I read the music, having been hearing the higher pitch on the trumpet, I read it wrong and sang an octave too high." Impressed, his teacher told him that, to allow the counter-tenor voice to settle, he shouldn't sing for a while. "Of course, I just went away and practised."
Music college was a disaster. "I was slung out in the middle of my second term. Some say my musical education began then," he says. He started lessons with Helga Mott - he stayed with her for 10 years - and within 12 months was earning a living, doing everything from deputising in church choirs, to pop sessions at Wembley Studios and radio jingles.
"The goals were to make a living and to make the sound as pure and straight as possible, because the majority of the work was ensemble singing. Now you can sing with a bigger, fuller voice, with vibrato, with much more vocal freedom."
As last year's jokey The Three Counter-Tenors disc shows, there is no longer one counter-tenor sound. Each of those three soloists has a distinctive timbre. While Chris was developing his sound in the wake of the 1970s counter-tenor boom heralded by the ascendancy of James Bowman - "a voice like a trombone," says Chris - his brother was studying singing at the Royal Northern College in Manchester and was having a struggle of his own. "I had a rather unfortunate obsession with Peter Pears," he grins. "I wanted to be able to communicate directly with people in the way he did. It wasn't so much that I wanted to make his sound, although I inevitably tried to, it was more to do with his understanding of humanity, a quality of compassion."
It was the Australian director David Freeman who brought them together and changed their lives. He had already cast Chris in his celebrated Opera Factory staging of Monteverdi's Orfeo, when the scheduled tenor, unable to cope with Freeman's dramatic demands, pulled out a week into rehearsals. Chris suggested Nigel, who had just left Manchester, and, after auditioning for Freeman and conductor John Eliot Gardiner, he too joined the company.
Chris credits Freeman with opening up his latent desire to improvise and perform, and the pair of them thrived. Nigel describes it as a shared, daring, idealistic desire to see how far Freeman's discipline of improvisation and characterisation could go in finding ways of speaking to an audience. "He created parameters for a performer, making us create a character who then played the role," says Chris. "That made it easier than just going in and playing Orfeo." They relished the shared responsibility for a piece, working with a director who liberated the performer, allowing them to discover things for themselves, something far more akin to theatre than the intensely formal, hierarchical world of traditional opera rehearsals.
The release of their dramatic powers ensured them distinctive operatic careers. They have both excelled in mad scenes, Nigel playing a powerful Madwoman in Opera Factory's production of Britten's Curlew River, while Chris's intense portrayal of Edgar and Mad Tom formed a still, emotional centre to Reimann's Lear at ENO. They played La Calisto together and, in a semi-staged concert version of the Britten Canticles, they put a fraternal spin on Abraham and Isaac. "Brother killing brother," muses Nigel mischievously. "Interesting."
Their religious upbringing has resurfaced, unbidden, in Theodora. For Chris, "one of the reasons I have been so forceful about Didymus the convert being so completely enraptured by his conversion is possibly a subconscious reaction to believing that this is a very real possibility." Nigel sees the religious parallel in wider terms. "One of the greatest gifts a parent can give to a child is a feeling of responsibility about making their own mind up. Not everyone has that. That feeling of freedom about religion lies at the heart of Theodora."
Whatever their thought-processes, the intensity of their scenes together in rehearsals has moved at least one observer to tears. Their next joint project may move audiences in yet another direction. Producer Jean Nicholson is hoping to present them in the title roles of an opera based on Genet's The Maids, specially commissioned from composer John Lunn. They are still negotiating the rights, but a 25-minute workshop of a couple of scenes has already yielded exciting results. As in Theodora, the intimacy between the characters is lent an extra charge by their own relationship: the epitome of sibling rivalry. How much more typecast can you get?
'Theodora': tonight, Tues, Fri, Glyndebourne Opera House, E Sussex (booking: 01273 813813), then touring