Fever can support the immune system's attempt to gain advantage over infectious agents, such as viruses and bacteria, and it makes the body less favorable as a host for replicating viruses and bacteria, which are temperature sensitive. Infectious agents are not the only causes of fever, however.
Your sister's grandson is your great nephew. You are his great aunt (if you are a woman) or his great uncle (if you are a man).
My sister, 14 years my senior, was having marriage trouble. She was divorcing, or already divorced from her husband who was an alcoholic, but having also his second child. His name was Andrew. Due to various kinds of stresses, my sister was extremely uninterested in Andrew when he was a baby, and unlike the first born, who was the first grandchild, Andrew was not celebrated.