Causes of sibling rivalry:

It depends on many factors like

Position in the family, for example, the oldest child may be burdened with responsibilities for the younger children or the younger child spends his life trying to catch up with an older sibling.

Sex, for instance, a son may hate his sister because his father seems more gentle with her. On the other hand, a daughter may wish she could go on the hunting trip with her father and brother.

Age, a five and an eight year old can play some games together but when they become ten and thirteen, they will probably be poles apart.

How to avoid sibling rivalry:

• Don't make comparisons. Each child feels he is unique and rightly so-he is unique, and he resents being evaluated only in relation to someone else. Instead of comparison, each child in the family should be given his own goals and levels of expectation that relate only to him.
• Don't dismiss or suppress your children's resentment or angry feelings. Contrary to what many people think, anger is not something we should try to avoid at all costs. It's an entirely normal part of being human, and it's certainly normal for siblings to get furious with one another. They need the adults in their lives to assure them that mothers and fathers get angry, too, but have learned control and that angry feelings do not give license to behave in cruel and dangerous ways. This is the time to sit down, acknowledge the anger and talk it through.
• Try to avoid situations that promote guilt in siblings. First we must teach children that feelings and actions are not synonymous. It may be normal to want to hit the baby on the head, but parents must stop a child from doing it. The guilt that follows doing something mean is a lot worse than the guilt of merely feeling mean. So parental intervention must be quick and decisive.
• When possible, let brothers and sisters settle their own differences. Sounds good but it can be terribly unfair in practice. Parents have to judge when it is time to step in and mediate, especially in a contest of unequal’s in terms of strength and eloquence (no fair hitting below the belt literally or figuratively). Some long-lasting grudges among grown siblings have resulted when their minority rights were not protected.