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).
So while Mother's Younger Brother's name doesn't announce its symbolism with spotlights and a megaphone like Mother or Father do, the name's symbolism is still very much there. Younger Brother is a sad dude who finds his identity in relation to other people: Evelyn, Coalhouse, and Pancho Villa. So it's fitting that his name itself shows his relation to, well, his relations. He's not himself alone; he's Mother's Younger Brother.