Here scribe 2 of 𝓟 initially wrote “ye are willing to repent of your sin” but virtually immediately added the plural s to give “ye are willing to repent of your sins” (there is no difference in the level of ink flow). Although the singular is theoretically possible, the Book of Mormon text has only examples of “to repent of one’s sins” (34 in all, including here in Alma 7:15); there are no examples of “to repent of one’s sin”. The corrected reading here in 𝓟 is undoubtedly the reading of the original manuscript.
Summary: Accept in Alma 7:15 the corrected reading in 𝓟, the plural sins in “to repent of your sins”.