Here Joseph Smith made two types of changes in his editing for the 1837 edition; the first deals with the relative pronoun which and the second with the number of the following be verb. Let us first consider the question of the relative pronoun. In this example, Joseph changed the which to who since the antecedent refers to humans (“the mixture of thy seed”). A similar change was made a few verses later; in this instance, the antecedent for which was the word remnant:
Nonetheless, the editing of which for these two nouns (seed and remnant) has been unsystematically applied in the text. In fact, Joseph’s first change of which to who (here in 1 Nephi 13:30) has never been implemented in any printed edition. More generally, the editing of which for the antecedents seed and remnant is uneven. Consider the following additional examples, of which only 7 of 13 examples have been edited to who:
For a complete discussion of this grammatical change, see which in volume 3.
We now turn to the second type of editing in 1 Nephi 13:30—namely, Joseph Smith’s changing of the number of the be verb from singular to plural (that is, from is to are). The form of the nouns mixture and seed (in “mixture of thy seed”) is singular, but its semantic referent is plural. Once more, however, this editing of subject-verb agreement with the noun seed has not been systematic. In the following example, Joseph changed the plural to the singular, the opposite of his change here in 1 Nephi 13:30:
For further discussion of this type of editing, see subject-verb agreement in volume 3.
Summary: Unless there is contrary evidence, we follow the earliest textual sources in determining the relative pronoun and subject-verb agreement in the relative clause; here in 1 Nephi 13:30, we restore the original “the mixture of thy seed which is among thy brethren”.