Short answers, pulled from the story.
The sister-wife of Njörðr remains unnamed in all surviving Old Norse sources. The Poetic Edda poem Lokasenna records Loki accusing Njörðr of begetting a son with his own sister, and Snorri Sturluson wrote Ynglinga saga describing how Njörðr took his sister as wife by Vanir custom.
Jacob Grimm published Teutonic Mythology in 1835 arguing that brother-sister marriage was an ancient Indo-European practice preserved among the Vanir. Joseph S. Hopkins published research on this figure in 2012 suggesting she might be the unnamed sister-wife named Njörun.
Tacitus wrote Germania in the first century describing rituals around a goddess named Nerthus whose name is etymologically identical to Old Norse Njörðr despite being female. Some scholars propose a single hermaphroditic deity split into two figures over time or suggest they were originally a married twin pair similar to Freyr and Freyja.
Ynglinga chapter four states Freyr and Freyja were born before Njörðr left Vanaheimr for Asgard while Gylfaginning section thirteen claims they arrived after he came to live with the gods. Skírnismál verse one calls Freyr our boy while Skaði speaks to him as if he were her child, creating a direct conflict about whether the twins existed before or after their father moved to Asgard.
Jan de Vries suggested this custom survived as a cultural remnant in early Germanic religion and Rudolf Simek noted that Tacitus described Nerthus as a female earth goddess whose name matches Njörðr. Georges Dumézil compared Hadingus' story in Saxo Grammaticus' Gesta Danorum to Njörðr's life noting parallels between foster-sister relationships and royal marriages.