Melancholia

The film begins with an introductory sequence involving the main characters and images from space and introducing many of the film's visual leitmotifs. These are virtually still images revealing the key elements of the film: Justine (Kirsten Dunst) the bride in deep melancholy with birds falling behind her; of a lawn with trees and a sundial with two different shadows; Pieter Breughel's Hunters in the Snow (often used as interpretation of an idealised nostalgia) the non existent 19th hole (limbo) and the Black Horse collapsing catastrophically in slow motion (id/ego battle); Justine as a bride being swept along by a river; and her being held back by her wedding dress; and finally Justine and her nephew building their magic cave before the earth crashes into Melancholia becoming one.
The film continues in two parts.Part One: "Justine" is of an ideal and perfect wedding that sweeps bride Justine along and which clashes with Justines fragile world.This act begins with an clumsy, oversized wedding vehicle unable to negotiate a curve in the road.Newlyweds Justine and Michael (Alexander Skarsgård) arrive two hours late to their own reception at the family estate where Justine's sister Claire and her husband John reside. Justine happens to notice a particularly bright red star in the twilight sky which John identifies as Antares.During the course of the wedding we realise that each of the characters represent a metaphor. These metaphors are developed during the night of the wedding reception.
Her brother-in-law, John, represents sacrifice (cost of the wedding) and social conformity.
Her dad is hedonistic and selfish therefore unable to connect at a meaningful level with Justine. Her mother ,Gaby (honesty) is brutally; ruthlessly and pathologically honest which gets her thrown out of the wedding. However Justines sister ,Claire, who represents order urges Justine to dishonestly hide her debilitating melancholy from her bland, gormless, doting husband, Michael, (This is highlighted by his wedding speech) and anyway who would be incapable of handling such honesty.Her boss represents ruthless greed and gluttony during the most personal part of the wedding speech he is hustling her to promote a vacuous spiritually empty campaign based on a modern facsimile of Bruegels The Land of Cockaigne (mythical land of excess) she later opens a book at this picture.
During the critical part of the wedding, cake cutting, Justine and her mother independently escape to have bath, to cleanse themselves of the social deception of the wedding.
Later again Justine escapes the wedding and whist watching Melancholias approach to earth squats and urinates on the 18th hole (ultimate success).Her bosses nephew (ambition) is given the opportunity to exploit the opportunity to get the tag line at all costs, to promote his career, a similar task to what Justine was previously so successful at. He is later fired for his failure at absolute ruthlessness. Justine recognising her complicity aggressively resigns. She cannot consummate her wedding with her gormless husband and goes out on a sand trap and copulates with her bosss nephew, a professional kindred spirit. He later suggests due to their similarity they form a business together, Justine curtly declines. Her gormless husband leaves.At dawn the next day, Justine reluctantly goes horseback riding with her sister Claire when she notices that the bright red star has disappeared.Part Two: "Claire" deals with Justines relationship with her sister, Claire It also follows Justines mental decay and reawakening as the inevitable Melancholia collision approaches.During Justine deepest almost catatonic depression her sister is unable to cleanse her in the bath, expressive of her inability to submit to any more social deception. Justine is so numb that even her favourite dish tastes of ash.Before the reception Justine proudly claims to be only one to ride the Black Horse(id); her brother-in-law under his breath says that he the horse is also loyal to him.As Justine decays her connection to the Black Horse (id) becomes more remote and frustrated. on two occasions the horse refuses to cross a bridge over a river possibly to the non existent 19th hole (limbo) as Justine disintegrates she get more brutal with her frustration with the horses refusal, finally, mercilessly whipping it to the ground.John explains that the reason for Antares' disappearance was the rogue planet Melancholia blocking the star from view. Melancholia, a large blue planet that had formerly been hidden behind the sun, becomes visible in the sky as it approaches ever closer to Earth. John is excited about the planet and looks forward to the "fly-by" predicted by scientists.
Melancholia's first approach and final collision with Earth, as described (and shown briefly in a similar diagram) in the film
Claire becomes very fearful that the end of the world is imminent in spite of her husband's reassurances that everyone will be safe. She searches the Internet and finds an article predicting that the movements of Melancholia around the Earth will bring the two planets into a full-on collision soon afterward.Justine tells her sister that she has the ability to predict with certainty events such as the number of beans in a bottle and she confident that Melancholia will meet with Earth, this being a good thing as life itself is evil, which from her tortured battle would be valid.The certainty that Melancholia is making its terminal rotation back to earth is at odds with the certainty of a safe fly-by her brother-in law has had, a fact that always been a reality for Justine.
On realising his fallibility Justines brother-in-law the only other person to ride the horse fatally poisons himself in the Black Horses stable the horse calms down. His wife on finding him dead releases the horse to be free.
In this adversity Justine becomes the spiritually the strongest person. She has a bath stating that she is now all clean.The sister tries to escape the golf course but the golf cart shuts down on the same bridge frustrating her. She returns to the lodge as the world begins its demiseHer young nephew being scared is reassured by Justine who says that they can be safe in a magic cave something she has promised several times in the film to make.The three sit in the magic cave (a wood tepee) Justine is stoic and strong at the as the world beautifully comes to a catastrophic end, and at one with Melancholia

Question: Who does Justine build a magic cave with?
Answer: Her nephew.