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).
As I say in an upcoming column that hasn't been released yet, it was a family feud. Cousins couldn't get along. The monarchs of Britain, Germany, and Russia were all related. So were many others in Europe. George and Wilhelm were grandsons of Queen Victoria; the Russian Empress was a granddaughter. It's slightly more complicated than that, of course. Since Napoleon European rulers had sought to ensure peace.