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).
Recently we had our first son, and my in-laws wanted to know what they should be called. My ingenous 11 year old sister-in-law came up with the names Amma- for the Grandmother and Adda for the Grandfather. She derived the names from lord of the rings when one of the Elves were talking.