It is quite normal for young kids to feel they are not getting the attention they need from their parents, especially if all of a sudden they have a baby brother/sister to take it all away. Even if your kids are of similar age, jealousy is most likely to happen when favouritism is shown or a parent is often busy or compares a kid to another.
When a baby is introduced to a kid as their new sibling, jealousy and mixed emotions will surface… and that is normal.