Actually, Aragorn's marriage to Arwen united the lines of Elros and Elrond. Elros and Elrond themselves were born of a union that united the half-elven lines begun with Luthien's marriage to Beren and Idril's marriage to Tuor. So when the appendices says
Quote:
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united the long sundered lines of the half-elven
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, it's referring to the half-elven Elros and Elrond (who, as the Mushroom pointed out, are actually 9/16 elven
).