It is good to take into consideration the social context for effective communication, as indicated. But I tend to always be specific about the ranking in the family, as well as mention the number of siblings (i.e. 1st of 9, 3rd of 5, 2nd of 2, etc.. ). Though it is anachronistic (usage of old linguistic style), I am able to have an understanding of certain social behaviors, aside from the previous parental instinctive structural bias towards their children.

I don't think that all human behavior can be easily interpreted through (modern) psychology, but this information can provide an explanation, along with the genders such as being first male or female, of a certain number of offsprings. Some character adjustment takes place by dint of birth order, this sort of data to me becomes even more crucial in cases where adoption is factored in. So indeed, more details as to "where I am coming from could help" the readers.