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).
(A Half-Sibling shares one parent biologically; a Step-Sibling is by virtue of one parent each entering a marriage, with no blood relation. Your Step brother/sister is already a child of your parent's new partner. Your Half brother is a new child born to your parent and the spouce.