If there is more than one child in a family it's usually inevitable to have teasing, arguments and sometimes even fighting. While your children may love one another, disagreements and competition among them is only natural.
The reason behind disagreements among these children is "sibling rivalry." When family has two or more children, the kids are naturally, even if unconsciously going to be rivals for their parents' love and attention.
For a young child it starts when there is the birth of a new brother or sister. The older child often sees himself or herself being replaced as number one in the parents' eyes. This child tends to believe that there is only so much love for a parent to give, and now there will be less love for him or her.
It's actually a rational fear growing out of the child's observations. An infant, out of necessity, is going to require more time and attention. Simply telling the older child that Mom and Dad can love all their children equally usually has little effect, since young children aren't capable of the abstract thinking necessary to understand this concept. The practical cure is to demonstrate your love for each of your children. Find time to spend with each child, doing something important or special to that child.
As children get older, it isn't jealousy over the amount of parental love but competition for parents' attention that can fuel sibling rivalry. Children will often compete, usually without realizing they're competing, to be "first" in their parents' eyes.
Children will work for better grades in school, display stronger athletic skills, develop musical or artistic talents, or even get into trouble if it makes Mom or Dad pay attention.
While sibling rivalry is a natural occurrence it shouldn't reach a point where physical or emotional damage is done to any of the children, or to the point where it disrupts family functioning. If a parent see one child "winning" all the time, they may want to tip the scales slightly giving the other child more time and attention.
All children have different levels of ability and a little friendly competition can often be a good motivator. But if that competition gets too serious, it can significantly harm a child's development of self esteem. That's when it's a good time to seek help from your school counselor or another professional counselor.