When copying from 𝓞 into 𝓟, Oliver Cowdery accidentally replaced the comparative elder with the superlative eldest. Since only two are being compared, the comparative is perfectly acceptable. The 1911 LDS edition grammatically emended eldest to elder, which happens to be the reading of 𝓞. Subsequent LDS editions have continued with elder, but the RLDS text has the textually incorrect eldest. For a full discussion, see under 1 Nephi 16:7.
Summary: Maintain in 1 Nephi 18:7 the original comparative elder, the reading of the original manuscript.