A child of normal parents can be a genius; siblings and children of a genius may or may not be geniuses. Genius is not a hereditary title. Francis Galton thought that genius was hereditary but he confused existence with opportunity and recognition. A family with one genius is more likely to recognize genius, and a wealthy well connected family will more likely provide opportunities for a child to do things that are recognized. Your parents might be poor; you might never have been given opportunities, but you can still be a genius.
How to be a genius