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).
Your aunt is the sister of your father or mother. Your great uncle (or grand uncle) is the brother of your grandfather or grandmother. Your great aunt (or grand aunt) is the sister of your grandfather or grandmother. Your great-great uncle is the brother of your great-grandfather or great-grandmother.