My 3-year-old brother has a bicuspid aortic valve. Is this something serious?

Maybe. The bicuspid aortic valve means it has 2 leaflets instead of three. Tis may not ever cause a problem. It can be hereditary. A cardiologist usually follows up on the condition. Aortic regurgitation can occur. Surgery is only sometimes necessary.
Yes. A bicuspid aortic valve is an abnormal valve. Typically, people with bicuspid aortic valve will develop valvular heart disease at a much greater rate than those without that issue. The problem also occurs earlier (usually in the 6th or 7th decade of life). While presently it is not likely to be a problem, in the future he has to be followed by a cardiologist.