Live: Supernal Fablers. CocoRosie, Royal Festival Hall.

Swimming through athletics screenings and scurrying across crammed foyers, it's surprisingly still once inside tonight's Royal Festival Hall. Indeed given the unpredictability of the inherently unorthodox CocoRosie, the evening could quite conceivably keep both serene and sombre: the sisters' proclivity for melancholia has and most likely always will be fairly pervasive, and as they hurdle the discography much woe remains. However that it be plenteously seasoned with joy differentiates this Meltdown showing from their richly varied – and with it variegated previous. Gone are the Sharpie moustaches, yet like stubborn infants clinging to unduly sugary desirables down the most wickedly calorific of supermarket aisles a sophomoric exuberance abides, Sierra Casady entrapped in compulsively perpetual pirouette as though perfecting some ultimately inconsequential preschool ballet routine. Let's just say you can but hope you're not merely acting but also enthralling thus at their age: Sierra's now thirty-two.

What of that repose aforementioned, then? Well, once they emerge they're greeted with an Ennis-esque (and given the unending stream of latecomer trickling down every aisle, perhaps Ennis-inspired) sense of hysteria and just as the duo tonight tinker with oeuvres plucked from their ever amassing history, they're aided by musicians seemingly picked up along the way. Like the clientele lining the leathered backseat of a retired hitchhiker finally come financially comfortable, they're a motley posse: superstar beatboxer TEZ, a considerably composed Japanese pianist seemingly aeons anyone and everyone's senior, and an enigmatic chap employed to attentively massage the lower end of the 'Hall's sound system formulate the siblings' backing band. With this outré assembly then paired off with Auto-Tunage, charming pungi aura aural and muffled operatics redolent of Underworld's Opening Ceremony compositions, permanent marker taches begin to assimilate into relative normality. For CocoRosie function on the raggedly hemmed fringes of society and sonic, dangling from sanity or whatever you may believe that to be. You've then two options: hang on in there for one of the more exhilarating rides of this year thus far, or loosen grip and fall back into televised mundanity. What with this being their first London date in over two years, you'd likely be wise to grasp the former.

From the anaesthetising swells of cheap synth and fuzzy brass parps of K-Hole through a luscious Tearz for Animals these sisters not only do it for themselves but do it through the mist of a previously unanticipated state of sangfroid. Of course it's a schizoid affair overall, and the roles and onstage appearances of the two are quite symbiotic for the most part but there's now composure to match their unequivocal songwriting class. And if spasmodically it'd appear to be two solo songstresses occupying either side of the stage then it's Sierra – or Coco – who steals this particular show: performing myriad stiletto manoeuvres worthy of the Opera House just across the Thames that're just as impressive as her ethno-soprano vocals, her Orient-infused atmospherics, fluttering harps and elaborate gesticulations are here confined to precisely the right sort of atmosphere. Too farfetched ever to be a triumphant festival act, our previous assignations have never felt quite right; even to book them in the first place is to tread a fine, tightrope-like line. But here everything glides into place like a perfectly executed swing aboard vertiginous trapeze. And crucially, their acoustics are perfect.

Daisy Chain, an eponymous piece from Bianca's current exhibition at NYC gallery Cheim & Read, proves a highly evocative gyp hop number: touching upon the sounds of a Middle Eastern caravan camp, Opera House aforesaid and a ghoulish, witch house-infested haunt all within one solitary song it not only intoxicates with an admirable instancy but also illustrates the sisters' ability to combine and fully collaborate. Undertaker too, the verbose fable of the apathetic mortician spun silk-like on an off-kilter pop spindle, exhibits their ability to align not merely genetically but also chemically.

Elsewhere though, this chemistry is eschewed somewhat in favour of what has become a quite burgeoning professionalism: Bianca takes the hip hop-slanted pop belter We Are On Fire to an incandescent climax; Sierra turns siren-like as she drapes layers of infinite loops atop irresistible allure on a prepossessingly undressed R.I.P. Burn Face. That all this plays out before visuals of slowly motioned corporeal mutilation (the vigorous swaying of heads, the primordial brandishing of sticks, etc.) enthuses as much as it is explanatory: this polarity of beauty and violence; of attraction and repulsion; of pleasure and pain which forever remains ambiguous and ultimately indecipherable mirrors the healthiest of sibling rivalries. For in love (whether familial or otherwise) lives jealousy, and resentment, and detrimental truth surreptitiously shovelled beneath façade. False moustache even. However in feuding may be found boon and there's no denying their adoration of and admiration for one another, and this creative disparity would appear to benefit the both of them. They're consequently perhaps best revered as two supremely, almost supernally gifted solo artists entangled in constant collaboration. For love or loathe, re: family your lumbered with what you're born into and for these two it's not just over the festal season that intimacy be thwarted: solitary has been irrevocably transposed with sororal solidarity.

Significantly less requisite to the evening's enjoyment is TEZ' beatboxing interlude: inevitable yet unspeakably irksome, it's not only a beatbox break but apparently a beer op for many also. I've never been one to go all in on Brixton's seemingly interminable B-Boy Championships; even on Medúlla, one of few instances in which I've felt it seem advantageous to outcome, it only functioned as accompaniment and never the main attraction. Well, the same is true of tonight. It incontrovertibly elucidates what has hitherto been only an intimation, yes, but hip hop has now become so inextricable from the work of CocoRosie that this segment feels rather too vainglorious. As talented as TEZ may be, decorated beanie to heel in guerrilla garb as he guides us through a viva voce encyclopaedia that's punctuated with sporadic eruptions of squeal, this be neither the time nor place. It's furthermore infuriating when contextualised by the positive buoyancy this and live bass lavish upon a sublime Fairy Paradise.

Thus with Sierra on harp and keys and Bianca on a variety of blowers, with both recited atop the odd slipshod sample the result is strangely symphonic: it's as though these five odds and sods equal one inexorable right. Bianca may be vocally inferior to her elder, but her loveably rough Animals is an ambling marvel bred with the sorts of rhymes that ought to be stringently taught at nurseries worldwide. (Albeit with those of "porno, and tea stains, and tobacco" abridged.) Then coming to represent a sort of Lewis Carroll-styled boozer band, the screw-loose cirque aesthetic is again adhered to although bizarrely interaction, irregardless of our patent obsessions, is kept to a minimum as the pair therefore retain their every ounce of inscrutable, nymphal mystique. Only on a rhythmless beat 'n' bass take on Kevin Lyttle's trashy dancehall smash Turn Me On, with the band at full brute, do they let loose. "So many divas tonight!" Sierra hollers as Nomi Ruiz and Yasmine emerge to gyrate anorexically to the childish charms of Be Your Side, her sardonic desires of ironing clothes, shining shoes, making beds and cooking food in compliance with the retrograde social stereotype of the archetypal "housewife" assuming an empowering F U humour. It's an astonishingly mean riposte to Beyoncé's superficial cross-examination of who, or rather which gender it is running the world contemporarily. Then, naturally, they unleash Werewolf.

It's far from tonight's most inspiring instant and is ultimately overwhelmed by support act stage invasion, yet its lyrics concerning the sisters' "schizophrenic father" – now an Iowan absent all but entirely enveloped in shamanism – reconnect an act oft regarded as freak, or otherworldly, or preternatural in one way or another with humanity and those "subordinate feelings" cast aside in Animals. Whilst they may not convey human emotion as we've come to expect our popular culture torchbearers so to do, herein lies their true artistry and with it their wholly symbiotic abilities: they are both unknowably abnormal and yet perceptibly anthropomorphic. And if a "love for humankind" may be reported to have prospered this past week with the Olympics' flexing of jingoistic muscle, never have I ever adored CocoRosie as unconditionally as I now do.