Is it at all possible that the spell linking Wanda and Jillian was a two-way street? Perhaps it was Wanda who initiated the spell, but that the spell creates a situation where either can influence the other. Perhaps Wanda really thought she was in control... because she has been LEAD to believe that by Jillian! Perhaps Wanda's refusal to believe Jillian would betray was actually a suggestion of her own to protect Jillian at whatever cost!

They were both double agents, and it became a question of who would break from the relationship first. Wanda was protecting Jillian, and fully expected Jillian to do the same for her. But Jillian broke the spell, and Wanda was hit by the snap-back.