] Does it make sense that Juliet would try to warn Caroline when her father had told her nothing? Why did she seem jealous if she knew that her father had forced the breakup?

Well, her father hadn't told her details, but she did know that he had disparaging information about Parrish.

You know, I don't think it was so much jealousy over Parrish as it was bitterness that it was Caroline, of all people, who was marrying him. I doubt Caroline was very gracious about it. Juliet accused her of being smug over "stealing" him from her - and smug she probably was.