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).
Yeah, and it is weird being the oldest and having a sibling that's close in age to your children. My oldest half-sister on my mom's side just turned 27 and the youngest on my dad's side is somewhere in the 16-ish area (I don't have much communication with them). My oldest child is 18.