Rivalry is a common behavior of Human being!. In your question, you asked about sibling rivalry is good or bad. It is the situation or the subject on which the rivalry start, is determine whether it is good or bad.
Sibling, not only as child, but also as teenage, as middle aged, as old aged, showing rivalry.
Rivalry start to get something from other one. It may be a book, fruit, food, money, wealth, or due to dis-opinion with the other. Such rivalry in many occasion, lead to bad result. It normally give negative energy and helps to spoil the life.
Rivalry in healthy matters, like score higher grade in study, in business and such creative thinking, will yield good result. This will generate positive energy and helps to achieve new goals in life.
Parent should always be vigilant and careful to guide and advice children to 'what to do and not to do'.
Family culture, parents behavior, the social culture, friends and friendship etc are the prime stone to develop character in an individual. Character and rivalry is closely related.
So best character development in childhood itself will lead to develop healthy rivalry in children.
Healthy rivalry promote healthy family, healthy family promote healthy nation.
Thanks for the healthy question you asked