A: That's because there is next to nothing. We don't know who her parents were, or very much else about her. The sum total of references to her in the Eddas are as follows:

Larrington:

"She saw a captive lying under the grove of hot springs,
That evil-loving form, Loki she recognised;
There sits Sigyn, not at all happy
about her husband - do you understand yet, or what more?"

Larrington:

"He (Loki) was bound with the guts of his son Nari. But his son Narfi changed into a wolf. Skadi took a poisonous snake and fastened it over Loki's face; poison dripped down from it. Sigyn, Loki's wife, sat there and held a basin under the poison. But when the basin was full, she carried the poison out... "

Hollander:

"They bound him (Loki) with the guts of his son Nari; but his son Narfi became a wolf. Skathi took a venomous serpent and hung it above Loki's face so that its poison dripped on him. Loki's wife Sigyn, sate by him and held a bowl under the poison, and she carried it out whenever it was full..."

Terry:

"He (Loki) was bound with the guts of his son Nari. But his son Narfi became a wolf. Skadi took a poisonous snake, and tied it up head downwards over Loki's face. Sigyn, Loki's wife, sat there beside him holding a bowl which caught the venom that dripped down. But when the bowl was full, she had to carry it outside..."

52:

"Then Loki's sons Vali and Nari or Narfi were fetched. The Aesir turned Vali into a wolf and he tore his brother Narfi to pieces. Then the Aesir took his guts and bound Loki with them across the three stones - one under his shoulders, one under his loins, the third under the backs of his knees - and these bonds turned to iron. Then Skadi got a poisonous snake and fixed it up over him so that the poison would drip from the snake into his face. But his wife Sigyn stands next to him holding a basin under the drops of poison. And when the basin is full she goes and pours away the poison..."

157:

"Now shall all the Asyniur be named. Frigg and Freyja, Fulla and Snotra, Gerd and Gefion, Gna, Lfn, Skadi, Iord, and Idunn, Ilm, Bil, Niorun. Hlin and Nanna, Hnoss, Rind and Siofn, Sol and Saga, Sigyn and Vor. Then there is Var, and Syn must be named, but Thrud and Ran reckoned next to them."