Jealousy and fear are part of the normal development of the child and they are generally overcome as time goes by.
Jealousy appears with the arrival of a sibling. The first-born child often fears that the parents will not love him any longer, will reject or abandon him.
These fears make him behave in a different way: he may act like a little child or try to act like an adult, he may get aggressive or capricious, or he may have different attitudes.
These are moments when the child needs special attention and affection, assuring him that he is still loved and also that there is another member in the family who will love and need him, too.
Remember that when a new baby arrives, it is normal for its siblings to get aggressive or regressive.
Talk to them, give them affection and protection.
Make them participate in the care of the new member and, why not, spoil them a little bit (not much) so that they do not feel left aside.