Byleth Der Damon Mit Den Blutigen Fingern (1972)

aka’s: Byleth (Il Demone Dell’Incesto)/
Byleth: The Demon Of Incest/
Trio Der Lust

The young Duke Lionello Shandwell (Mark Damon) loves his sister Barbara (Claudia Gravy). When he learns Barbara has married Giordano (Aldo Bufi Landi), he goes into a jealous rage and tries to break up the union. Meanwhile some mysterious murders terrorize the village–the victims are all women, killed with a three-point blade. (In Italian language, with English subtitles).