When the age gaps between siblings increase, there is a likelihood of a "second family." In your case you are biologically the youngest of the family, so you were probably looked after quite well by your parents and your elder siblings.
Because the age gap is 7 years between you and your brother, you don't belong to that "family" anymore and instead form a second branch of birth order, which makes you an only child. I would say you are probably have a youngest child personality with only child tendencies.
In the case of your brother, he was the youngest son for quite a while before you came along, so it's natural that he developed a youngest child birth order personality. Usually by 5 most of our personality is already developed. When you came along he couldn't be the baby of the family anymore, and instead had to be an elder sibling to you, which sorta made him feel caught in the middle. So I'd say he's a youngest child with middle child tendencies
So it is possible for someone to have a combination of 2 birth order personalities.