In this verse, Joseph Smith revised the subject-verb agreement for the 1837 edition so that seed would be treated as a singular (thus “our seed which is a remnant of the house of Israel”). He was perhaps influenced by the singular subject complement that follows the be verb (“a remnant of the house of Israel”). Earlier in his editing, Joseph made the opposite change in subject-verb agreement:
For further discussion, see under 1 Nephi 13:30. The critical text, of course, will maintain the original plural are in 2 Nephi 28:2 but the singular is in 1 Nephi 13:30.
Summary: Restore the plural are in 2 Nephi 28:2 (“our seed which are a remnant of the house of Israel”).