The original manuscript is not extant here. In this particular case, spacing between extant portions of 𝓞 is sufficiently long that it is difficult to tell whether unto or to would fit best in the lacuna. Oliver Cowdery initially copied the text into 𝓟 as to; then he crossed out the to and supralinearly inserted unto. The correction appears to be virtually immediate since there is no change in the level of ink flow. Elsewhere in the text, there are ten occurrences of “brought (un)to X”, where X is a person, and in all cases but one (which is marked below with an asterisk) the preposition is unto:
Either unto or to is possible, but the archaically styled unto is preferred. For a list of places in the manuscripts where Oliver Cowdery accidentally wrote to in place of unto, see under Jacob 2:17.
Summary: Accept in Alma 57:6 the corrected reading in 𝓟 of “brought unto us”.