Short answers, pulled from the story.
The sister-wife of Njörðr is an unnamed goddess in Norse mythology who was Njörðr's twin sister and first wife. Together they were the parents of Freyr and Freyja, according to the 13th-century sources Ynglinga saga and the Eddic poem Lokasenna.
She appears in two 13th-century sources: the Eddic poem Lokasenna and the Ynglinga saga, composed by Snorri Sturluson. She receives no further mention in any other Old Norse texts.
Snorri Sturluson's Ynglinga saga explains that Njörðr married his sister because sibling marriage was the custom among the Vanir. This practice was particular to Vanaheimr and is presented as a Vanir social norm rather than an aberration.
The name Nerthus, used by Tacitus in his first-century work Germania for a female earth goddess, is etymologically ancestral to the Old Norse name Njörðr. Scholars have interpreted this to mean that Nerthus and Njörðr were originally a married divine twin pair, with the sister-wife of Njörðr being a surviving trace of that earlier female deity.
Njörun is an obscure goddess name that appears in skaldic kennings and in the Nafnaþulur appended to the Prose Edda. Some scholars have proposed that Njörun may be a surviving name for the otherwise unnamed sister-wife of Njörðr, based on the pattern of Norse divine pairs sharing differentiated but cognate names.
No, the sources contradict each other. Ynglinga saga and Lokasenna place the birth before Njörðr left Vanaheimr, making the sister-wife their mother. Gylfaginning and the poem Skírnismál place the birth after Njörðr joined the Æsir and married Skaði, attributing parentage to that later union instead.