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).
The child of your cousin is NOT your second cousin, an error some people compound even farther by declaring that the grandchild of one's cousin is therefore a third cousin. No, no and no! The child of your cousin is your child's second cousin.