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).
Grandma, you have some boundary issues. You are not the granddaughter's parent, and she is 18, a legal adult. You should not do or say anything to come between your daughter and her daughter. Your daughter is the mother, not you.