"... at times she was strange and dangerous. But mostly she was sweet and scared and harmless."
-- Aberforth Dumbledore (DH28)
Ariana Dumbledore was the younger sister of Albus and Aberforth – Albus’s junior by four years, Aberforth’s by one. At six years old, she was attacked by three muggle boys and was severely traumatized by the event. Her father, Percival, was imprisoned for seeking revenge on the boys. As a result of the attack, Ariana’s magic was too unstable to send her to Hogwarts. Her mother, Kendra, moved the family from Mould-on-the-Wold to Godric’s Hollow and hid Ariana to keep her out of St. Mungo’s.
Years later in 1899, Ariana accidentally killed Kendra during one of her rages. Albus was forced to put his world tour on hold to take care of Ariana, even though Aberforth (who claimed to be her favorite) offered to be her caretaker. Albus, Aberforth, and Grindelwald had an argument about the responsibility of taking care of Ariana, which turned into a violent duel, where Ariana was killed by a stray spell. She was buried next to her mother in Godric’s Hollow under the epitaph “Where your treasure is, there will your heart be also.”
Ariana was then forgotten by history, except in the Hog’s Head, where Aberforth kept a portrait of her for the next century. She was brought to the fore of the news cycle when Albus died, being mentioned in Elphias Doge’s obituary of Albus and in Rita Skeeter’s salacious biography, The Life and Lies of Albus Dumbledore, though neither source knew the full story of Ariana’s tragic life. Her portrait played a small but key role in the Battle of Hogwarts by providing passage from Hogsmeade to Hogwarts via the Room of Requirement.
Unable to control her magic due to trauma.
Cf. Beth March from Little Women by Louisa May Alcott. -BB
There is a popular fan theory that Ariana Dumbledore might have been an obscurial, and there have even been several articles written about this theory. While we do not know for sure or have any confirmation regarding this theory, it certainly seems plausible. Ariana had been attacked by a group of muggles when she was very young and as a result she became unstable and had magical outbursts. It's possible that her "outbursts" were the same kind that Credence experienced, after all, why would her family be afraid that she'd get sent to St. Mungo's for being a threat if Ariana was simply having magical outbursts typical of young and inexperienced wizards like the ones Harry had before going to Hogwarts? Considering she had been forced to stay hidden and wasn't allowed to attend Hogwarts, it wouldn't be an outlandish possibility for her to have repressed her magic to such an extent that an Obscurial could be formed. --Miles