What happened to Albus Dumbledore's sister?

Answer 1

When Ariana Dumbledore was six, a group of muggle boys say her practising magic in her back garden and demanded that she do the same thing again. Ariana was unable to do this and the boys violently assaulted her leaving her emotionally scarred for life and unable to control her magic. Afterwards, Ariana was forced to hide in her home to avoid being sent to St Mungo's leaving many people to believe she was a Squib.

When she was fourteen, Ariana caused an explosion which killed her mother and as her father was in Azkaban for taking revenge on the boys who attacked her, it was left to Albus to take care of her - a task he was not happy to take on as it meant putting his life on hold.

Albus then became friendly with Gellert Grindelwald and together they planned to find Deathly Hallows, reveal the wizarding world and change the world so that wizards ruled the muggles. This caused many arguments with his brother Aberforth who despised the ideas and Grindelwald. One day, the three began duelling and Ariana got caught up in the middle where she was killed by one of the three duellers.

After this, Grindelwald ran away and ended his friendship with Albus who always regretted that day. Nobody knows who actually cast the spell which killed her, something that haunted her brothers for their entire lives.