In principle, every human being is selfish because most people would protect their own self-interest no matter the cost. But in a family environment, sharing forms a basis of a person in the society and the unwillingness to share would have adverse effects, particularly among siblings. In this context, sibling rivalry is a hard issue to tackle and if the competition between 2 siblings reaches an alarming point, then the whole situation could be harder to solve. Therefore, one could use the act of sharing to solve the problem of fighting for attention between the children. The parents would have to be fair in this context.
Therefore, if the act of sharing is well instilled among the children, then it would have a positive effect on the rivalry. In fact, if brothers and sisters are aware that they must share their food, their space, their toys, their books with each other, then it takes away the pressure to be better than the other. Furthermore, it will also change the selfish attitude which they might have. The child would also be able to understand the need to protect their siblings from being bullied by others when the situation arises. Sharing is caring is a cliché but it is true to its very meaning.