Responses
it is a sequel because the detective Damian, who is investigating the death of Kuroiwa's wife , first appeared in the other stigma series and he didn't know about Minami ,but in this he already know about Minami.
http://www.mangago.zone/read-manga/stigmata_seikon_sousa/uu/stigmata_seikon_sousa-chapter-4/10/ - first series
http://www.mangago.zone/read-manga/stigmata_aikon/uu/stigmata_aikon-chapter-1/39/ - second series
This is a prequel not a sequel !