Here in the printer’s manuscript, Oliver Cowdery initially wrote “the Lamanites whom ye hate”; later he supralinearly inserted “your Brethren” with heavier, but uneven, ink flow. It appears that he used a defective quill to make this correction in the printer’s manuscript. There would have been nothing wrong with what he originally wrote; if anything, the corrected reading is unusual and awkward for English, which argues that the phrase was in the original manuscript (no longer extant here).
There is at least one other example in the text of an appositive intervening between a relative pronoun and its antecedent:
In this example from the book of Alma, Joseph Smith edited the relative pronoun which to who. Yet even for which, its antecedent is clearly “the Lamanites”.
Summary: Maintain in Jacob 3:5 the appositive “your brethren” that follows “the Lamanites” and precedes the relative clause that modifies “the Lamanites”.