I begin with few points:
1) It's interesting that the first episode where Kennett appeared, "Crazy", was the first appearance of Cece but also the first appearance of the Children's ward in Radley.
2) Marion Cavanaugh was a nurse. In some way, Jessica and Marion are involved in something big (a Radley's mystery). Peter Hastings helped Mrs D for that. Bethany is part of a Radley's mystery too (maybe the same). Melissa's riding helmet was found where Jessica used to take Bethany.
3) In the books, the Hastings used a surrogate mother to have Spencer. The show loves to adapt parts of the book's story by changing little details; so in the series, another character, (maybe Melissa) could have had a storyline close to a surrogate of even about adoption. Also we just learnt Andrew was adopted...
4) Jessica was afraid of what the Hastings are capable of. She had an agreement with Peter, which we never heard the details of; we speculated it was about Spencer. Jessica had a terrible secret for Kennett, a secret linked to the yellow dresses. What if the agreement was about Melissa ? What if the Hastings are involved in whatever happened to Charles. Jessica knew, but not her husband, at least Kennett knew only one part of the story.
5) Do you remember the Hastings flashback in "Unbridled" ? Veronica and Spencer talked about the summer before Ali's disappearance. That's when Spencer began to take drugs. Why ? It looks like it's because of an "incident" between Spencer and Melissa. Spencer has no memories of this events. Is it because of the drugs or because of schocking informations about her sister.
6) In 5x25, "ChArles" wanted to recreat the prom of Ian and Jason senior year. Cece was queen of this prom, but in the episode only Ian and Melissa were mentionned. Was it a clue ?
Here the theory. If Jason believed "Charlie" was an imaginary friend, it means that Jason was really young when something happened his brother. The DiLaurentis lived in Atlanta during Jason childhood if I remember well. But the video showed that the family used to have some trip, especially at the Campbell farm. A place where the Hastings went too. I believe the characters around Jason's age are involved in Charles story. It's not only the NAT club, a terrible thing happened when they were 4 or 5 years old. I think Melissa did something to Charles. But it wasn't the end of the drama. In 6x01, we heard Radley was shut down, I think Radley is linked to a big scandal, involving children or even babies. I suspect that Marion helped Jessica to find out some messed actions done by the Rosewood medical staff (illegal adoptions, babies taken away); that maybe Charles was switch at birth. The one who died wasn't her biological son. Jessica knew Kennett was still grieving, she decided to stay quiet about it until she found where his son really was. I think Jessica and Marion found a lot of bad secrets, bad enough to drive Marion crazy or at least depressed. I speculate, Melissa, Charles, Bethany, Andrew, and maybe Toby and Cece too, all passed by this children's ward when they were kids. Jessica revealed some details to Peter, because of Melissa, but I believe, whatever she found about the real Charles, she kept it for herself. So Peter know nothing about Charles. Now the question is, who is the real Charles DiLaurentis ?
About Cece and Melissa. They seem to share a link. In season 3, it was implied that they knew something about "that Night", and were lying about this. In 4x24, both share informations about that. Marlene said, one was telling the truth when the other was still lying. It seems they both have good and bad sides, and the 2nd part of my theory is that one of them is truly evil. A lot of us, fans, thinks that Cece is against the Liars while Melissa was only protecting Spencer. What if it's what the writers want us to believe, and in fact it's Melissa who is the crazy one ?! I don't think she could kill her sister, but she could have killed everyone else in Rosewood ! In 3x24, she called the girls, "bitches"; she wanted really badly to get back the NAT club videos, she faked her pregnency after Ian's death... She scared me, I can't trust her !