[Finwe]

The Dark Elf, you are rather mistaken. The term "Westernesse" refers to the island of Numenor itself, not to the people who lived there.

Look at it this way, sometimes genetic traits skip generations or siblings. For example, if your father has black hair and your mother has blond hair, you can have black hair and your sibling can have blond hair, but still be your sibling. The inheritance of the "Blood of Westernesse" works the same way. By the time of the Third Age, it was so diluted, especially in Gondor, that it probably skipped generations and siblings like crazy. Denethor had it, but he used it to look into others' minds and judge them harshly. Faramir had it, but he used it to understand others, not judge them. Unfortunately, Denethor loved Boromir more, and the fact that Faramir was that much more "powerful" than his brother probably contributed to the bitterness with which Denethor treated Faramir.