I am the daughter in this situation and growing up I did indeed feel quite left out although I think that could really have been minimised by my parents.
In my family there was my oldest brother - 3 years - my next brother - 4 years - me.
So for me I think being the only girl was more significant than the age gap.
However I will say that my brothers were very much treated like they were the same age and were always seen as a package deal (“the boys”) whereas I was treated as significantly younger.
For example, when it came to gradually being allowed independence growing up (being allowed to stay out later, go to bed later, stray further from home when playing outside etc) they were allowed to do everything at the same time as though they were seen as the same age but I was seen as too young.
When we went away on holidays for example they’d go off exploring together but I was seen as too young to go out without my parents and by the time I could have gone with them, my role as the annoying younger sister had been firmly cemented.
There isn’t a single thing I can think of that my oldest brother was allowed to do that the middle brother was not.
We didn’t have a bad relationship by any means but they did get a lot of praise from my parents for being patient with me, for entertaining their little sister etc.
It was as if they were told from the start that being around me was a chore and the default was that I was annoying and they had to “put up” with me.
They wouldn’t ever have praised my oldest brother for example for letting the middle “tag along” with him because the narrative was they were both on an equal level.
Looking back I really think the way everyone spoke about them made a big difference. The age gap between me and the middle child wasn’t much more than the gap between the middle and oldest child but that isn’t how it felt in my family.
As adults, we became close, although the middle one has drifted in recent years and I mostly spend time with the oldest.
Dont get me wrong - it wasn’t the end of the world but if I could give advice to anyone it would be to be careful what roles you put everyone in because of assumptions.
Its perfectly fine for some siblings in the family to have closer bonds - whether because of age, gender or personality - but I think we massively underestimate how much adult assumptions and the way they speak play into that.