Short answers, pulled from the story.
The word Sif derives from the plural Old Norse term sifor which denotes kinship or affinity by marriage. Scholars trace cognates across Germanic languages including Gothic sibja and modern English sib meaning family connection. Rudolf Simek defines it as relation by marriage within his 2007 Dictionary of Northern Mythology.
Snorri Sturluson wrote the Prose Edda in the 13th century describing Sif as the loveliest of women with golden hair while listing her ancestors as unknown. He notes Thor married her and mentions Ullr as a son of Sif and stepson of Thor in chapter 31 of Gylfaginning. Most scholars continue to consider the father unidentified since neither Edda names him.
Loki cuts off Sif's hair as a prank according to the story recorded in Skáldskaparmál when Thor discovers this he grabs hold of Loki forcing him to swear an oath. Dwarfs known as the Sons of Ivaldi create the golden headpiece for Sif alongside Odin's spear Gungnir and Thor's hammer Mjöllnir. This narrative appears in chapter 31 of the Prose Edda book Gylfaginning.
Jacob Grimm proposed a reconstruction of a Germanic deity cognate to Sif during the 19th century connecting Eddic references to Sif's golden hair with the herb name Polytrichum aureum. Guðbrandur Vigfússon theorized in 1874 that Sif betokens mother earth with her golden sheaves of grain while H. R. Ellis Davidson states Sif may have been an ancient fertility goddess agreeing with links to wheat fields. Rudolf Simek rejects notions of a vegetation cult venerating Sif in his 2007 work.
Sif has been linked with Ravdna the consort of the Sami thunder god Horagalles where red berries of rowan were holy to Ravdna and the name resembles North Germanic words for the tree. Old Norse reynir refers to the same plant mentioned in Skáldskaparmál as salvation of Thor who once saved himself by clinging to this tree during battle. Richard North notes sib is personified here suggesting references to Sif in Danish religion.