I'm on baby number one so I can't quite answer your question, however, I do have 3 sisters!!
My older two sisters are 7 and 5 years older than me from my mums previous marriage and my third sister is my little sister Becky who is 15 months younger than me. Growing up it was very much them vs us and it wasn't until me and Becky were more adult that we all really got on.
When I was younger I did get annoyed that when my mum finally deemed me old enough to do things on my own for example, I had to wait until I was 14 to go into town with my friends on my own, it also meant that Becky was allowed to do it as well. Which, to a 14 year old, didn't seem fair lol
But now, having 15 months between me and my little sister is great. She is more like my best friend. I think that we had a fair few arguments because we were so close in age and would fight over who got to wear what top and silly stuff like that, but on the other hand it meant that we were always there for one another at school (she was a year below me) and we liked all the same things etc.
She really is my best friend, and I don't think that would of happened if she'd of been 3/4 years younger than me. She's going to be my maid of honor when I get married
The same happened to my older two sisters, they're best friends as well as sisters. (Not that we're not all close now but you know what I mean)
Anyway, long story short, I think an 18 month age gap will be perfect