Brent Kerby (personal communication) suggests that at the end of this verse the original text could have read “the mixture of thy seed which is among the seed of thy brethren”. He points out that elsewhere in Nephi’s vision there are 16 instances of “the seed of my/ thy brethren”. From a logical point of view, Nephi’s seed (or the mixture of his seed) would be among Laman and Lemuel’s seed. Nonetheless, outside of Nephi’s vision, there are two other instances like the original text here in 1 Nephi 13:30:
In both cases, the text does not read “with the seed of thy brethren”, even though semantic or logical consistency would technically require it. Also notice that the instance in Alma 3:15 refers to the mingling of seed, just as here in 1 Nephi 13:30 there is a reference to the mixture of seed. The critical text will therefore accept the earliest reading in 1 Nephi 13:30; the use of “thy brethren” rather than “the seed of thy brethren” appears to be intentional.
Summary: Maintain in 1 Nephi 13:30 the reference to being “among thy brethren”; usage elsewhere in the text shows that this is not necessarily an error for “among the seed of thy brethren”.