from my own experience. my brother is 3yrs younger than me and my sister 2 yrs older. my sister was a nasty spiteful bully growing up who put me down and bullied my friends. me and my brother were like best friends, always together, played out together and had same group of friends. I move out the family home at 18 and we lost the connection, my sister and I get on a little better now at 35 & 38 but wouldn't say i miss her.
from having my own children. my ds's are 3yrs apart and hardly ever got on when they were younger as they got older they came to blows with punching, throwing objects and shouting. they are 12 and 15 and cant stand each other. when ds2 was 6 I had dd. at first he adored her, looked out for her - got her toys, put dummy back in etc. pushed her on the swings at park. soon as she started to walk, talk and become annoying in his words he stopped caring and started hitting, pushing and being nasty. ds1 hasn't really made an effort or had any interest in her. dd doesn't like her brothers just as much! 
I have a friend who has 2 boys also 3yrs apart and they too don't get on, sit in separate rooms and get into smacking fights.
I have a neighbour who has a 5yr old dd only child, she is the happiest little girl I know, the parents are always taking her to events, family gatherings, shes always kept busy.
I think it depends on your own situation, if you have lots of family and friends / mum friends then might be ok with socialising that way. only you will have the gut instinct I reckon.