It looks like a useless burden and a waste of energy, why it evolved?
First, it must be understood that not all traits in an animal are evolved or selected traits – some are only variations. Some of these variations may be selected by some evolutionary pressure in the future and then become evolved or selected traits.
For example, each of the various hair colors (black, brown, blond, red, white, etc.) is not an evolved or selected trait – all of them are only variations in hair colors. For now, there is no evolutionary pressure to select any of them, so present-day people have various hair colors. But suppose there is a strange disease that gradually kills off people who do not have blond hair; then the blond hair will be selected by this evolutionary pressure. And after some time, there will be only people (or the majority of people) with blond hair. At this stage, the blond hair will become a selected trait. These also happened to the variation of the finch’s beaks in Galapagos, some of which were selected by evolutionary pressure in some specific ecological niches. This pressure led to speciation that resulted in various kinds of finches that Darwin found in various niches. So were true for variations in appendages, which at first were in in-complete forms but later were selected by evolutionary pressure in various ecological niches and evolved into arms, wings, and fins.
How do we know that a trait is an evolved/selected trait or a variation of a trait? One important characteristic is that, if it’s an evolved/selected trait, it will appear in all or in the majority of that species. So, for example, the human hand, the bird wing, and the fish gill appear in all humans, birds, and fish – these are evolved/selected traits.
Yet, not all traits that are present in all/the majority of a species always are evolved/selected traits. Some may be just vestigial traits or just results of genetic drifts (see Evolution’s Useless Traits). But these are exceptions rather than the general rules.
The general rules of evolution are variations in traits, competition, differential production/survival among different traits, and heritability of (some) traits (Natural selection, Natural Selection – Wikipedia).
For the case in question, siblings jealousy/rivalry that is strong or destructive does not seem to be an evolved/selected trait because it does not occur in all humans and not in the majority of human families. It’s probably just a variation trait in some families. And minor jealousy/rivalry is not an evolved/selected trait either. It is probably just one emotion of several emotions that the versatile amygdala (and other emotion-creating neural centers) can produce (to flag various kinds of stimuli that an animal encounters). The amygdala is the evolved/selected trait; any of the various emotions (including sibling jealousy/rivalry) that it can produce is not.