Through Nephi, God tells Joseph Smith not to touch the sealed text—the revelation to the brother of Jared (Ether 3:25–27). The “seal” on the brother of Jared’s revelation is symbolic, not physical, otherwise it seems likely that at least one of the eleven witnesses would have described a physical impediment to viewing all of the individual plates. (See commentary accompanying Ether 5:1.) After the witnesses have seen the plates, the plates will be returned to the Lord and hidden again, thus “sealing” the entire book.
Translation: These three verses, along with verse 20, interrupt the story, change the speaker from Nephi to God, and change the audience from the generic reader to Joseph Smith personally. Perhaps God interrupted the translation to dictate these specific instructions to Joseph Smith, either expanding the text he was translating or interjecting new text that was not on the plates. In other words, God would be using Nephi’s text just as Nephi used Isaiah’s—as a springboard to revelatory communication. Of course it is equally likely that the text was literally part of the prophecy. There is no way to tell, but neither interpretation of these verses violates the general integrity of an inspired translation of an ancient text.