Osbourne was killing people with Thallium and using the witches as a smokescreen. He had sent out the cards to people who might want someone dead, telling them to say the name of the person to the witches at The Pale Horse. He used Jessie, to speak to the witches and find out who had visited them. As such, the witches were entirely innocent in the scheme, and only showed up to funerals after realising that they were being used.
Jessie realised Osbourne had started to poison her so she wrote the names of who would die – and included Mark as his wife, Delphine, had visited the witches. However, it becomes clear that Delphine had met with the witches because she simply wanted her fortune told.
Osborne also poisoned Hermia and Sean as per Mark's request – but doesn't manage to kill Hermia. When she awakens from her coma, the witches are sat by her side, presumably to tell her that Mark had finished them wanting her dead. Mark then returns home and reads a headline announcing his death - then has a vision of the moment he kills his beloved first wife Delphine in a jealous rage. The supernatural twist revealed that the witches really did have powers, and it is indicated that they killed Mark and cursed him to relive Delphine's death.