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).
If you share a common ancestor you are cousins. So she is not nothing to you, she is a blood relative and part of your family tree. If your mother has a sister who has a daughter, you and the daughter would be first cousins. If your grandmother has a sister who has a grandaughter, you are second cousins. If your great grandmother has a sister who has a great granddaughter, you are third cousins.