Hi, I'd like to help you with your question.
It sounds like your granddaughter's parents are handling the temper tantrums very well.
When a child acts out with a tantrum, the best option is to be sure they are safe and do not pay any attention to the behavior. When the tantrum is over, her parents can talk to her about her behavior (which is sounds like they might be doing already) and help her find better ways to handle her frustration in the future. One of the ways could be teaching her options for expressing herself. For example, the next time she feels upset, she can go in her room and yell into her pillow. Or her parents could buy her a punching bag that she can use to vent her frustrations.
One of the reasons your granddaughter may be having temper tantrums is her high intelligence. When children have a high IQ, they can become frustrated when they are unable to express themselves in a way that is satisfying to them. It would be much like you trying to say you are angry but you can only say two sentences and use only simple words.
As for your granddaughter having schizophrenia or suicidal tendencies, tantrums are not a precursor to either of these problems. Many children have tantrums. They are simply ways a child handles frustrations. Nothing more. The symptoms of schizophrenia do not manifest themselves until the late teens or early adulthood. And suicidal tendencies usually have more to do with environmental and emotional issues rather than heredity. While it is true that suicide is high in some families, there are usually underlying issues such as alcoholism and abuse as well.
Let me know if I can help any further,