How did God make Nephi's brothers loathsome to the Nephites?

By making their skins black.

He [God] had caused the cursing to come upon them, yea, even a sore cursing ... wherefore, as they were white, and exceedingly fair and delightsome, that they might not be enticing unto my people the Lord God did cause a skin of blackness to come upon them.

And thus saith the Lord God: I will cause that they shall be loathsome unto thy people. 2 Nephi 5:21-22