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).
Step siblings are the children of your step parent. So, if your mom married a man, and that man had a daughter from a previous relationship, she would be your step sister. A half sibling is biologically related to you through only one of your parents.