Now I rather thought the description in the Lightbringers book is enough, and if you don’t have a copy your should order it right away!
But let’s assume you have that book. What the Thunder Brothers you still want to know?
First, don’t try to be overly precise. The Thunder Brothers are best thought of as a band of storm gods that follow Orlanth around as companions, followers, manifestations, messengers, etc. They are violent and aggressive, armed with lightning and thunder and swords, and shake mountains, split clouds, destroy forests, flood rivers, and the rest.
Ever been in a really wild thunder storm with loads of things going on? Torrential rain, clouds so dark that it looks like night came early, walls of lightning, rolling thunder, and winds that knock over trees and structures (I call that Tuesday afternoon)? Orlanth has brought Thunder Brothers over. Many have well-known names, many have names that are really just titles or descriptions (“loud roarers,” “impetuous ones,” “glory-seeker”, “pounding rain”, and so on). Vinga is said to be their leader, but they are all the “Sons of Orlanth” (even if female), and serve the storm god.
Calling them bullies both understates what happens in a storm and overstates bullies. The best description of the Thunder Brothers I have is they are what shows up when the National Weather Service gives a “severe storm warning” in my part of the country. They are the gods that throw semis off the freeway, tear off roofs, shatter trees, cause flash floods, and start huge fires. Bullies? They are way more dangerous than that. Sure they eventually go away but only because they got called away. Offer them sacrifices and hope they rampage more on the other side of I25!