As already noted under 1 Nephi 3:29, there are a few places in the original text where the historically singular second person pronoun thou (or thee) is used to directly refer to more than one individual (such as “Laman and Lemuel” and “mine elder brethren” here in 1 Nephi 7:8). Such instances have been consistently edited out of the text (in this case, in the 1840 edition, presumably by Joseph Smith).
Such mixtures of singular thou and plural nouns within the same clause are not found in the King James Bible. The biblical style deriving from Early Modern English permits the use of the historically plural second person pronoun ye and you for singular referents, but not thou and thee for plural referents. In the original Book of Mormon text, this occasional use of the second person singular pronouns for plurals suggests that thou (and thee, thine, and thy ) can be used for either singular or plural. Its more generalized usage in the Book of Mormon seems to be an indicator of the biblical style. Such examples like “thou art” here in 1 Nephi 7:8 will be restored whenever there is support from the earliest textual sources. See the discussion under thou in volume 3.
Summary: Restore in 1 Nephi 7:8 the historically singular “thou art” even though it is used with the plurals “Laman and Lemuel” and “mine elder brethren”.