Albus Dumbledore significant person Ariana Dumbledore
Harry Potter and the Deathly Hallows
Rowling used several chapters in Harry Potter and the Deathly Hallows to reveal two major details concerning Dumbledore: his early life and his death. The book introduces his parents, Percival and Kendra Dumbledore, as well as his little sister, Ariana; his brother, Aberforth Dumbledore, was mentioned in previous books. At six years old, Ariana was attacked by three Muggle boys who had witnessed her performing magic. Because of this attack, Ariana was seriously traumatised and never able to control her magic again. Enraged, Percival attacked the Muggle boys (killing them in the film version), and was given a life sentence in Azkaban. After this, Kendra moved her family to the village of Godric's Hollow. In one of her outbursts, Ariana accidentally killed Kendra at around the time that Albus completed his education. Albus became the head of the family and was forced to remain in his house with his sister while Aberforth completed his education.Backstory
Through various interviews and discussions about the character, Rowling has established that Albus Dumbledore was born in July or August 1881 to Percival and Kendra Dumbledore. The character had previously referenced his brother Aberforth, and in Harry Potter and the Deathly Hallows, it is revealed that Albus is three years older than Aberforth. The same book introduced their sister Ariana who, at six years old, suffers a vicious attack by three male Muggle youths who had witnessed her doing magic. Frightened, they first try to get her to repeat what they had seen, and upon her failure to recreate it due to her young age, they try to stop her from being "different". The severe trauma of the attack leaves Ariana unable to function socially or to properly perform magic again. It is not explicitly stated in the novel what form this attack took, but Percival is given a life sentence in Azkaban for tracking down and hexing the boys who traumatised his daughter. To prevent her being institutionalised in St. Mungo's Hospital, or hurting someone accidentally with her uncontrolled magic, Kendra moves the family to Godric's Hollow, and conceals Ariana's illness. Their wizarding neighbours and acquaintances assumed that Ariana is a non-magical squib, and that Kendra is hiding her out of shame.
When Dumbledore and his friend Elphias Doge leave Hogwarts at the age of 18, they plan to take their "then-traditional" tour of the world. On the eve of their trip, however, Ariana accidentally kills Kendra during one of her uncontrolled outbursts of magic. Because Dumbledore's parents are absent (his father in Azkaban and his mother dead), he becomes the head of the family and it becomes his duty to put food on the table, as the family's misfortunes have left them poor. He is forced to remain in his house with Ariana while Aberforth completes his education. Aberforth knows of his brother's resentment and offers to care for Ariana himself, claiming that she prefers him to Albus, but Albus refuses, stating that it is his duty as the eldest child to care for the family.
When Gellert Grindelwald arrives at Godric's Hollow, he and Dumbledore become immediate friends, and the two dream of a new world order in which wizards rule over Muggles "for the greater good". Dumbledore and Grindelwald suggest that they have to leave the Dumbledore home to carry on their plans, but Aberforth protested against this, as Ariana's condition would not allow her to be moved. The ensuing argument between Albus, Aberforth and Grindelwald turns into a duel, and as Ariana becomes more and more stimulated, she is caught in the crossfire and killed - though neither Albus nor Aberforth are sure whether one of them or Grindlewald caused her death. Stricken with grief and remorse, Dumbledore breaks ties with Grindelwald for good. According to the author, as a result of this, Dumbledore's boggart becomes Ariana's corpse. In Philosopher's Stone, he also mentions to Harry that the deepest desire of his heart, revealed by the Mirror of Erised, is to have a pair of woollen socks, but in the seventh novel, Harry realises that he and Dumbledore see the same thing in the Mirror: their reunited families.