I hope you don't mind me commenting.
My girls are 11 and 7 years old and seperately they are delightful children. When they're with their own friends they're lovely. When they're with each other they're very different
My eldest hasn't much patience for her little sister. My little one gets really cross that she can't do the things her big sister can do and I'm sure that they must lie in their beds at night plotting ways of getting the other into trouble. I spend a great deal of time mediating and encouraging household harmony which works for about a minute... before they revert back to winding each other up.
When my eldest was an only child life was so calm and I think that she would rather have my devoted attention of the time. When my youngest is alone with me she is a differentl child and I feel that she would've benefitted from never having to share me with her sister.
So, if I could wave a wand and have a look at how our lives could've been.... I'd say that both of my children would've probably dealt very well with being only children