I think the treachery that occured could still be loosely considered as "kin unto kin". The faithful sons of Bór were distantly akin to the sons of Ulfang, because they were all Swarthy Men. In Chapter 20,
The Silmarillion says that:
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Great was the triumph of Morgoth, and his design was accomplished in a manner after his own heart; for Men took the lives of Men...
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Anyone who joined with or came into contact with the exiled Noldor (Teleri, Edain and presumably also Swarthy Men) joined the ranks of the cursed. Thingol brought his own doom on him when he spoke of his desire for a Silmaril, and of the Edain it was said:
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All these were caught in the net of the Doom of the Noldor...
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I think that Nilpaurion Felagund is right; the treachery that took away all hope of victory in the Nirnaeth was a result of the Curse of Mandos.