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I don't think you should feel ashamed about this. You can't control what you dream about. I'm also one of those people who say that dreams don't have to mean anything. Sometimes they do, but not in all cases. I don't think it means that you actually want to engage in incest with your parents.
Has anything out of the ordinary happened lately, like seeing anything pornographic, or maybe a more "intimate" scene of a show or movie came on the TV while you were with your parents? Or, has anything different happened with your parents lately that may be causing you to have these dreams?
Maybe it will help to set a different mood to go to sleep, so you have more pleasant dreams? For example, set a calming environment with your favorite soft pillows, blankets, stuffed animals, etc. You can use aromatherapy like nice lotions so you have something pretty to smell. You can even do progressive muscle relaxation or guided imagery (the guided imagery may be especially helpful) so you fall asleep to something relaxing. There are videos on Youtube for this.