Hela is the goddess of death in Norse mythology, but is not Thor's or Loki's sister. In fact, Ragnorok is caused because Loki and Hela had several children together (all monsterous, Fenrir the Wolf is the most famous one), each capable of consuming the world. Loki is imprisoned and forced to suffer eternal acid dripping onto his face for not killing his children (Odin's orders), but only contained them. Interestingly, Loki's wife (I don't recall her name, but definitely not Hela), holds a cup to catch the acid, but when it fill completely, she must empty it, accidentally allowing acid to hit him. When Ragnorok occurred, Loki would then break free, and freed his children, and allowed Hela to cross into Midguard (the Earth) and Asguard (Home of the Gods), causing death and destruction. The Norse gods fought those monsters Hela, and Loki, and all of them were killed, including Thor. The Earth survived, and humanity survived. Ragnorok isn't like most end of the world stories, because all it does is kill the gods, monsters, and most of humanity. The vikings considered themselves the survivors of Ragnorok.
Oh, and Valhalla isn't referenced at all. Odin prepared for Ragnorok by taking the best warriors to his grand hall, to prepare for the end of days.
