Here we have a passage where there has been grammatical variation between elder and eldest, the archaic comparative and superlative forms of the adjective old. A similar example is found later on in 1 Nephi:
The Book of Mormon text always uses the biblical forms elder and eldest, never older and oldest, but the question in these two passages is whether the comparative should be restricted to two and the superlative to more than two.
In 1 Nephi 16:7, the 1830 typesetter replaced elder with eldest, undoubtedly because of the grammatical preference for the superlative form when more than two are being compared. Ishmael obviously had more than two daughters since earlier in this verse the text explains that Nephi and his brothers (here Laman, Lemuel, and Sam) married daughters of Ishmael. All subsequent editions have continued with eldest.
In 1 Nephi 18:7, Oliver Cowdery copied the grammatically correct elder as eldest, yet only Jacob and Joseph are being discussed. The 1911 LDS edition restored the grammatically correct elder, which happens to be the reading of 𝓞. Subsequent LDS editions continue with elder, but the RLDS text has retained the textually incorrect eldest. Note, in particular, that the original text here in 1 Nephi 18:7 conjoins elder with younger (that is, both conjuncts are comparatives).
According to Merriam-Webster’s Dictionary of English Usage, the superlative has been used for comparing two throughout the history of English (see under superlative of two). And, in fact, the Book of Mormon text refers to the two sons of Helaman as “the eldest” and “the youngest”:
An extreme desire for grammatical correctness might tempt one to change these three superlatives in Helaman 3 to their corresponding comparatives, but this would be wholly unnecessary. The use of the comparative elder in 1 Nephi 16:7 goes in the opposite direction. Here the semantics clearly require the superlative (“eldest daughter”), but we get the comparative instead (“elder daughter”). There is one example of an edition replacing the superlative with the comparative:
Without a doubt, this change in the 1920 LDS edition was grammatically motivated. (See the discussion under Mosiah 9:2.)
Thus this use of the comparative elder in 1 Nephi 16:7 is unique within the text, which suggests that this reading of 𝓞 is an early textual error: perhaps Oliver Cowdery accidentally wrote down elder when Joseph Smith dictated eldest, or maybe Joseph himself incorrectly read off elder instead of the correct eldest. What may have motivated the error here is the preceding word brethren:
In the text, we get occurrences of “elder brethren”, never “eldest brethren”:
In fact, this collocation also holds between elder and the related noun form brother(s):
In other words, there are no examples of “eldest brother(s)” or “eldest brethren” in the text.
This same collocation holds for the adjective young. There are no examples of “youngest brethren” or “youngest brother(s)”, only “younger brethren” and “younger brother(s)”:
The point here is not that the superlatives eldest and youngest are restricted from occurring with brethren or brother(s). Instead, the text simply has examples of only the comparatives elder and younger with brethren and brother(s). This means that given the previously dictated word brethren, the following eldest could have readily been replaced by elder because of the frequent collocation of elder with brethren (and more generally, the frequent collocation of the comparative with brother(s) and brethren). In other words, the preceding occurrence of brethren could have prompted elder rather than the correct eldest, leading either Joseph Smith to dictate “elder daughter” or Oliver Cowdery to write down “elder daughter” (rather than the correct “eldest daughter”).
Summary: Accept in 1 Nephi 16:7 the 1830 compositor’s emendation of elder to eldest (thus “Zoram took the eldest daughter of Ishmael to wife”); the incorrect elder in 𝓞 seems to be have been prompted by the frequent collocation in the text of elder with the word brethren.