Speak openly and honestly with your child about his behavior. Calling attention to it and telling the child how that behavior is negatively affecting your own life may be enough to curtail it in the future.
While the "terrible twos" and adolescence are often considered when discussing difficult time periods for parents, the problems can continue into adulthood. As the child gets older, the parent's authority often wanes, especially once the child is out of the house and living on his own. This isn't necessarily a bad thing, but it does make it more challenging when dealing with an adult child who is jealous of either a new spouse or some other change in your life. Dealing with a jealous adult child requires patience and discipline.
Spend time with your child and the object of the child's jealousy on family excursions. If you've recently remarried or started dating again and the adult child is resentful, it can be helpful for her to get to know the other person. This can help jealousy to subside, especially since she is being included in activities.
Limit the attention you give your adult child when jealous behavior starts getting problematic. By not indulging the child when she engages in jealous acts or comments and then being more receptive once the behavior stops, you send a clear message to the child about what isn't tolerated. Your adult child will then need to modify her behavior if she wants to keep you in her life in any significant way.
Talk to a family therapist if the jealousy problems continue and disrupt your life. Try to get your adult child to go with you as well. A trained professional can help get to the root of the jealousy, identify what behavior is and isn't healthy and help you both move forward in a productive way.