I’m from Uruguay and my English is not very good but I’ll try: Since season 1, the character’s development of Van Pelt seemed suspicious to me. I don’t think she is Red John because I believe he’s a man and older than Grace. But RJ could be someone related to her…
Reasons:
Just in the first episode of the series there are a lot of hints about Van Pelt but attention was directed only to creeps Brett Partridge and Dr Wagner.
- Van Pelt is the only one in the team who for sure arrives after Patrick. She’s the new one.
- Those gestures she makes when she wants to look like a good and sweet school girl. At the beginning seemed hers but now she uses them when she wants to manipulate someone. 
- How her face changes when she gets angry with Jane. Her comments telling him: Poor sad man, etc. Jane told RJ “sad, sad man”.
- Her dogmatic beliefs in psychics (cousin Yolanda), Red John wrote to Jane: “If you were a real psychic…”, he also believes in them.
-The comment Jane made to Wagner: Why magicians have beautiful assistants? To distract attention! Van Pelt is red haired and beautiful. She’s distracting everyone.
-How could Jane know her father was football coach just from her demeanor? Suspicious… maybe he knows her, recognizes her. Also, Jane told her she believes in Yolanda because she let her talk with someone who died and VPelt loves. (Her sister who committed suicide…, maybe because of Jane?)
Well, now I have to go but i'll come back with more reasons.