According to a study by The Leipzig University, there actually is a sibling who's born to be the smartest.
Apparently, the eldest sibling is the smartest!
According to the study, they get an IQ boost from having to teach their younger brothers and sisters how to do things.
The study also suggests that this comes from the undivided attention the sibling receives while they're the only child.
So, basically, if you're the eldest child or an only child you should be a genius! No pressure, though...
But if you're the youngest child, I wouldn't worry too much... Remember what Lisa Simpson figure out.