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).
Offer a special memento for a grandchild's wedding. Grandparents are a prime source of the something old that is supposed to be part of the bride's ensemble. Perhaps you have a lovely piece of jewelry that you could loan to the bride or groom for the wedding. Make the offer, but don't be hurt if it is not accepted.