Hello BBB2, I dont think that you are one bit unintelligent. Stop measuring yourself against others. I know that it is so difficult to do especially when the people that you are measuring against is your siblings. There are different ways that intelligence is not measured. Some people with high IQ have no common sence. I am sure you know them. I went to school with this guy who was as bright as a button and went on to become a doctor. Well done him I say, but he had no communicational skills whatsoever. After all his years in medical school he left and he now works in a lab, which suits his personality better. There is always someone that will be more intelligent, more beautiful etc... but being intelligent does not mean that they are in any way better than you. Work on your self asteem, you feel inferior, but that may have nothing to do with intelligence.
Take care of yourself.