Introduction
Sif is a goddess, wife of Thor, celebrated above all for her magnificent golden hair, the loss and dwarf-forged replacement of which set in motion the legendary contest that produced several of the gods' greatest treasures, including Thor's own hammer Mjolnir.
Family
Loki's prank
As a mischievous prank, Loki secretly cut off all of Sif's famous golden hair while she slept. Thor, enraged at the insult to his wife, threatened Loki with violence unless he made amends, and Loki was forced to seek out the dwarves to craft a solution.
The dwarf-forged hair
Loki commissioned the sons of Ivaldi to make Sif a head of hair from real gold, so fine and skilfully wrought that it would take root and grow naturally like living hair once placed on her head. This commission led directly to Loki's further wager with the dwarf brothers Brokkr and Eitri, resulting in the creation of several more treasures of the gods, including Mjolnir, in the same extended episode.
Family
Sif is the mother of Thor's daughter Thrud; sources differ on the mother of Thor's sons Magni and Modi, with some naming Sif and others the giantess Jarnsaxa.