Could you tell me what's the chance of my future kids inheriting my brother's heart defect?

Not very likely: Parent to child inheritance of any form of congenital heart defect (not necessarily the same) runs about 4%. Uncle to child is much less likely as these issues involve sites on several different gene sites (multifactoial). The risk of some unexpected outcome to any pregnancy (cleft palate, prematurity, infection, etc.) runs about 4%. Chd in the general population runs ~1/500.