Some rivalry between siblings is normal and healthy, as each child competes to define who they are as an individual. All children need to discover who they are, and as they go through this process of finding out what their talents, activities and passions are, they want to show that they are separate from their siblings.
Sibling rivalry can also occur when children feel they are being treated unequally in the way you provide affection and discipline. Sometimes sibling rivalry can become a problem, and be very wearing for parents! The good news is that there are ways of managing it.