Oliver Cowdery accidentally changed the singular father to the plural fathers. The 1981 LDS edition restored the correct reading. In this passage, Nephi is explaining to his brothers the meaning of their father’s prophecies about the house of Israel, mentioned earlier in 1 Nephi 10:
Elsewhere there are 13 places where the text refers to the Spirit being in someone. In each case, the reference is to a single individual, not a group of people. In fact, there are five examples with the same general phraseology as 1 Nephi 15:12 (that is, “the Spirit of the Lord which is/was in X”):
Thus the change to the plural fathers in 1 Nephi 15:12 is inconsistent with how this phrase is otherwise used in the Book of Mormon.
Summary: Maintain the singular father in 1 Nephi 15:12; the passage obviously refers to Lehi, not Nephi’s ancestral fathers.