According to Donald Parry, in 1 Nephi 21:12 the land of sinim may refer to Syrene, in southern Egypt, also known as Aswan, where there was a large Jewish colony after the Exile. Or it may refer to the Desert of Sin, which is in the peninsula of Sinai. The point is that the Lord will gather his people from wherever they have been scattered. [Donald W. Parry, Understanding Isaiah, p. 430; see also Thomas R. Valletta ed., The Book of Mormon for Latter-day Saint Families, 1999, p. 64]
However, we find the following in the the LDS Bible Dictionary: "It is uncertain what country is meant, but it must have been one of the most distant lands known to the writer of the prophecy. China has been suggested, but the identification is uncertain." [The Church of Jesus Christ of Latter-day Saints, LDS Bible: Dictionary, 1984, p. 775]
Note* If the land of Sinim is indeed China, then it appears that there was some sort of information regarding this land which was had by the Jews at least by the time of Isaiah. Whether this testifies of some sort of maritime contact which was useful to Lehi and Nephi is unknown. [Alan C. Miner, Personal Notes]