In response to Arwen and Aragorn, I don't think it made a big difference. Look at this quote (referring to Maeglin and Indril)
Quote:
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He loved the beauty of Indril, and desired her, without hope. For the Eldar wedded not with kin so near, nor ever before had any desired to do so.
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Note the 'not with kin
so near' part. That implies that if the gap were distant enough, it wouldn't matter.
As far as Aragorn and Denethor, are you sure they were even related? I've only read the Silmarillion once, so correct me if I'm wrong, but I seem to remember that the only men in Gondor that came from Numenor were the descendents of Elendil and a choice few others. Denethor came from a long line of Stewards (he was the 26th, right?) and as far as I can remember, had no blood relation to Elendil. He was just filling in.
Feel free to correct me if I'm wrong on any of these points.