I have read that siblings' IQs are within 10 points of each other. My little one is very sweet and seems totally normal. My older one is a bit edgy, uses big words, and likes to be the expert (even when he has no idea what he is talking about!).
Once in awhile, the little one seems very bright- he has memorized each word of several 50 page star wars books. Other times, he will say "I goed" instead of "I went." He's 6.