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).
First you find the common ancestor. If you share a parent, you are siblings. If you share a grandparent, you are cousins. Great-grandparent = second cousins. Great-great-grandparent = third cousins. So your grandma and her cousin have a common ancestor in a grandparent. Your grandma's grandparent is your great-great-grandparent. So you and your grandma's cousin (and the cousin's children) are third cousins (because you share a great-great grandparent). Then you have to work out how many times removed you are.