I grew up in what I believe was the worst of 2 worlds. I'm not complaining because I tend to be a loner, so being alone suits me. As a young child I had 4 siblings who were much older than me. MUCH older. They picked on me and made fun of me almost continually, and it was so nice when they all finally left home to go on to school, etc. I do not think having these unpleasant experiences made me a better or happier person.
So while I am the much younger of 5, there was a point where for many years I was the only child left at home. My parents not being terribly social and being very strict, I was not allowed to go much of anywhere but to school. We lived in the country, the closest village being about 4 miles away. Nothing to do around there. There were times when it was lonely, but I would only have to think of how it was when my sibs had been at home to appreciate my aloneness.
My daughter is an only child. I think she has moments of selfishness like most honest people realize they do, but she also has many moments of selflessness when she gives and helps others. I mention in some of my hubs on home school how she volunteered for various charitable organizations from the time she was just 11, and it was her own idea to do so. She can be very generous of heart.
My daughter and I are both very innovative, hard working, and generous as well as empathetic and compassionate towards other people. I do not think having siblings makes a lot of difference in cultivating these virtues. It is probably genetic, and while environment always plays some part, I think having sibs is not a necessity in order to be a well rounded, worthwhile, contributing member of society.