Scientific facts about siblings
Sixty-five percent of mothers and 70% of fathers exhibit a preference for one sibling over another.
Firstborns are generally smarter than the younger siblings, having on average, a three-point IQ advantage over the second sibling.
Younger siblings tend to be more extroverted than older siblings in large families.
In general, siblings interact significantly less with each other if they are not fully related.
Have more older brothers may have an impact on sexual preference.
The number of siblings you have and your birth order can influence your health.
Birth order does not affect personality.
Siblings tend to resemble each other in looks and intelligence but are quite different in personality.
Source: nursing schools.net