As it seems, the spell Charles put on Cassie's granma has a few side effects on her memory.
Jake helps Cassie finding a hidden family tree within those papers she got earlier. He runs off to tell the news to his witch hunter buddies that Cassie is actually a direct ancestor of Balcoin, a dark super witch. Even though the hunters want Jake to kidnap Cassie, he tells her what he knows about her ancestors while the others find out about Jake being a hunter.
In the end, the circle manage to free the kidnapped Cassie with the help of Jake, who finally admits that he has feelings for her. Jake stays with the hunters and leaves on their boat but not before we learn that there is another child of Blackwell, ancestor of Balcoin within the circle.
So, who is it? The most obvious one is certainly Faye. According to Cassie's granma, Dawn was obsessed with John Blackwell and I personal would find it funny if Faye and Cassie at some point figure out that they are actually half sisters.
But Faye might be a tad too obvious. Adam is raised by his father, who was in love with Cassie's mother. I find it hard to believe and maybe a bit too cruel if he got 'cheated' by John Blackwell not once but twice.
Diana is also living with her dad and though we don't know much about her mother, I can't see Charles being the type to get blindsided by a cheating wife.
Melissa is, compared to her cousin, quite fair skinned so there might be a connection, or not. The last one, Jake, isn't it. Why would the hunters try to kidnap Cassie if they already got a Blackwell ancestor to exploit?
Just my two cents.