Why is my sisters shark killing her guppies and a neon tetra?

I dont think its a bala shark. sounds more like a redtail shark or rainbow shark both look very similar. These sharks can get agressive with tank mates and each can have its own "personality" Your may just be a more mellow one. They are however semi agressive with tank mates that are smaller than them so that may be why its attacking the guppies. If you put the two sharks in the same tank they also tend to fight with others of the same species. The only thing i can recomend doing is adding some more hidin for the fish to retreat to and may help them from getting picked on. Or find a new home for the shark.