My brother has long qt syndrome. How great is my (and my kid's) risk of having it also?

50% Lqts is due to a genetic abnormality of one of the channels that moves electrolytes across the cell membrane of heart muscle cells. These defects typically are inherited in an autosomal dominant fashion. It is possible that your brother has a de novo mutation. If your brother inherited lqts from one of your parents, though, you have a 50% risk. Your son's risks are dependent on your status.
Get an ekg. All should be screened, 55 bucks at santa rosa health dept.