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).
My niece and nephew call my grandmother GG-she is their Great-Grandmother, and that's what it stands for. I call my grandmother Grandma, but as I understand it in the modern age, kids have all sorts of pet names for their grandparents. Gigi might be one of them. I have also heard Meema... can't think of any others.