"My older brother has a fever."
Why isnt there a 了 here?
In all other sentences here they add the 了 when someone is sick.
We can include 了 to indicate it has happened. And it is not wrong to state this fact with a sentence like — 我的哥哥发烧，并非只是感冒 “My brother has a fever, not just caught a cold.”
Why is there no "yo" before fever for the "has" part of the sentence? A few questions earlier when asking if someone had a headache it included "yo mei yo".
“有 + action” means the action has been done (at least once, as it could be still ongoing).
Literally, if 发烧 is translated as “have a fever”, then 有发烧 is “have had a fever (and maybe not recovered yet)”.