What chance does a sibling have of acquiring add if two brothers have already been diagnosed?

Probably good: Add comes in all degrees. Some are more severe than others and others are mild and don't need medications. Setting up a good foundation of organization and calm environment with minimal stimulus during focus activity is extremely helpful.

Not just genetic: Add/adhd does indeed tend to run in families (i.e., to "be genetic"). But there are also environmental factors that can really make a difference. Do your best to make healthy lifestyle choices -- limit your intake of refined foods, sweets, and foods with chemical additives (food dyes, preservatives, msg); engage in regular aerobic exercise; adopt good sleep hygiene; vitamin b complex; fish oils.

Inherited disorder: ADD is an inherited disorder. The incidence is separate for each pregnancy independently. So a third child in the family has the same likelihood as each of the other brothers.