As discussed under Helaman 13:22, in the Book of Mormon text the preposition in can be lacking at the head of a relative clause when it is immediately preceded by a prepositional phrase headed by in, not only here in Ether 13:15 but also in two other cases:
In order to deal with the awkwardness of the earliest reading in each of these three cases, the 1920 LDS edition added a preposition before the relative pronoun which, either in (in 2 Nephi 2:22 and Ether 13:15) or with (in Helaman 13:22). For another case of this kind of grammatical editing, see under Ether 11:21, where the preposition by was editorially inserted in a similar situation. Here in Ether 13:15 the critical text will restore the earlier reading without the preposition in before which.
Summary: Remove in Ether 13:15 the textually intrusive preposition in before the relative pronoun which; the earliest text lacks the expected preposition here, but this reading, however awkward, appears to be intentional since such usage can be found elsewhere in the original text.