In my discussion of this passage on pages 521–527, I argued that the original text had the indefinite pronoun one, so that the second line cited here actually read “I will raise up one unto the fruit of thy loins”. As part of that discussion, I should have noted one particular textual variant that entered the 1858 Wright edition and was repeated in the first two RLDS editions (in 1874 and 1892), namely, the replacement of the indefinite article the with the object pronoun thee. Although this change was very likely a typo introduced into the text by the 1858 typesetter, it is interesting that this reading provided a direct object for the clause, namely, the nonspecific “fruit of thy loins”— as if the text read “I will raise up fruit of thy loins unto thee”. Of course, the 1858 reading is rather bizarre, and the 1908 RLDS edition restored the earlier reading to the RLDS text (but without the conjectured one, of course). Elsewhere in 2 Nephi 3 there are 18 instances of “fruit of one’s loins”, and all 18 are preceded by the; there is also one example in Jacob 2:25, and it too is preceded by the (“the fruit of the loins of Joseph”). The 1858 introduction of thee here in 2 Nephi 3:18 is clearly secondary and quite impossible textually.
Summary: Maintain the original definite article the in the phrase “the fruit of thy loins” in 2 Nephi 3:18 and elsewhere.