Here in verse 20, the 1849 LDS edition replaced their with thy, probably accidentally. The preceding “thy loins” in verse 19 was undoubtedly the cause of the change. The first printing of the subsequent 1852 edition was set from the 1849 edition, with the result that it continued the 1849 reading of “thy brethren”. This change was reversed (probably by reference to the 1840 edition) when the 1852 stereotyped plates were revised for the second printing of that edition.
Summary: Maintain the original reading “unto their brethren” in 2 Nephi 3:20.