As discussed under 1 Nephi 2:5, the scribes sometimes mixed up brothers and brethren or sometimes the spelling of brother(s) was influenced by brethren. For these two examples in 1 Nephi 3, the original text has been maintained despite some difficulties the scribes had in transmitting the text correctly. For one other example, see 1 Nephi 13:10.
In the first example listed here (1 Nephi 3:28), the unknown scribe 2 of the original manuscript incorrectly wrote down the singular Brother rather than the plural Brothers. The context clearly shows that the plural is correct (“unto us their younger brothers”). In the printer’s manuscript, Oliver Cowdery initially wrote Brethren, but then he erased the final n and overwrote (with heavier ink flow) the first e with an o and the erased n with an s. The resulting Brothres stands for brothers, which is how the 1830 compositor set the word.
In the second example (1 Nephi 3:29), the unknown scribe 2 in 𝓞 correctly wrote the singular Brother. Oliver Cowdery copied this into 𝓟 as a plural, spelling it as Brethers. His choice of the plural was undoubtedly influenced by the plural of the preceding “their younger brothers” in verse 28. But Oliver caught his error here in verse 29 and overwrote (using a slightly heavier ink flow) the first e with an o and crossed out the plural s at the end, thus ending up with the singular Brother. The 1830 compositor correctly set the word as brother.
Summary: In 1 Nephi 3:28–29, the context requires the plural “their younger brothers” in verse 28 but the singular “your younger brother” in verse 29.