Who is Nienor from Unfinished Tales of Numenor and Middle-earth and what is their importance?

Nienor is the second daughter of Morwen and Hurin. Nienor was not born until Turin was sent to live with the elves. The siblings never meet until after Glaurung cast a spell on Nienor. She and Turin marry and conceive a child without ever knowing of their real relationship to one another. When the veil is lifted from her eyes, she realizes what she has done and is unable to deal with the guilt. Nienor jumps to her death rather than facing the consequences of her actions.