Roberto is of course right; I had the two characters the wrong way round, though it's worth mentioning that Ewan MacColl collected a version in which the Brown Girl was the heroine, and the Fair Girl had the money. The singer in that case was a Traveller, incidentally: that version is in the Database, here. To revise my original post, then:

Fair Ellender (Eleanor), who has no property, appears to conform to the then-current ideal of (aristocratic) beauty; the "Brown Girl" does not, but she does come with "houses and land". This looks, then, like a case of the nobility marrying into a nouveau-riche family in order to restore decayed fortunes -a common enough thing. The song goes back at least to the 17th century; A.L.Lloyd & Ralph Vaughan Williams observe, in the Penguin Book of English Folk Songs, "...most of the English versions, and all the numerous American ones, obviously derive from a broadside text published during the reign of Charles II and often reprinted."

Malcolm