As noted in the discussion under 1 Nephi 10:10, the Book of Mormon consistently uses the plural sins to refer to the atonement of Christ except when the text specifically refers to John the Baptist’s declaration of Jesus as the Christ as recorded in John 1:29 (namely, only in 1 Nephi 10:10 and 2 Nephi 31:4). Here in Alma 5:27, the 1874 RLDS edition introduced the singular sin, but the 1908 RLDS edition restored the correct plural to the RLDS text.
Summary: Maintain the plural sins in Alma 5:27, the reading of the earliest textual sources; the Book of Mormon consistently uses the plural sins rather than the singular sin except when explicitly referring to John the Baptist’s language.