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).
He is your brother-in-law. There has already been a similar question posted here to which the answer was sister-in-law, so I'll finish the list here: Your spouse's brother or your sister's husband is your brother-in-law. Your spouse's sister or your brother's wife is your sister-in-law. Your spouse's father is your father-in-law.