Here we have a complex conjoining of noun phrases in a prepositional phrase beginning with of. In the printer’s manuscript, the initial of was not repeated for any of the subsequent noun phrase conjuncts. But the 1830 typesetter added the repeated of to the second noun phrase conjunct (“and of the seed of thy brother”) but left the third one unchanged. However, this third noun phrase conjoins with the preceding thy brother and not with the earlier noun phrase the seed (in other words, the text is equivalent to “and the seed of thy brother and the seed of they which shall go with thee”). There is nothing wrong, of course, with the repeated of; on the other hand, it is not necessary either, so we follow the earlier reading here in Ether 1:43.
In terms of grammaticality, the subject pronoun form they could be replaced in the standard text by those, the appropriate object pronoun form when followed by the relative pronoun who: “and those who shall go with thee” (the current text reads “and they who shall go with thee”). Elsewhere in the text, Joseph Smith frequently edited examples of they which (and them which) to those who. For some discussion, see under 2 Nephi 1:5; for a complete discussion, see under pronominal determiners in volume 3.
Summary: Remove in Ether 1:43 the intrusive of that the 1830 typesetter added before “the seed of thy brother”; also maintain the subject pronoun form they and restore the original which in “and they which shall go with thee”.