Isn’t the translation of a niddah, any woman who a man is forbidden to? I assumed Niddah to be an adjective (that she is a niddah towards other men) but you just translated it as a noun (that it becomes her identity).
The word you are looking for is “ervah” if the cause is due to familial relationship. I’m not aware of a word (other than “forbidden” itself) to describe a forbidden relationship not due to familial relationships.
The Wolf