The quietest people I know have been younger siblings. The louder people I know have been older siblings or only children. I'm not sure why that is. Probably a coincidence.
I personally am an only child, and have, in the past, scored very low on extraversion (even though I'm DEFINITELY an extravert, and am energized by social activity and such). But honestly, the fact that I'm an ESTJ only child of an INTP and an INFJ consistently confuses and intrigues me. Dunno how that happened.
My mother is an Extrovert.
My brother's father is an Introvert (He had very little to do with my brother, though).
My father is an Introvert (He had nothing to do with my upbringing).
My brother is an Introvert.
I'm an Extrovert.
I always felt sorry for my brother - living with two extroverted women without a buffer couldn't have been very easy. But he's really quite charming and sociable for an I because of it, I think (he's even the president of his fraternity ), and I tend to come across as shy in unfamiliar situations while I'm trying to get my bearings. So we both tend to confuse people at first!
i feel like being an only child had a negative effect on my social skills. maybe because i never made that much of an effort to make friends.
I think for me it was kinda the opposite. I have two sisters pretty close to my age that I've always gotten along with, so I never really needed to look for friends; mine were built in. I have other (also introverted) friends who are only children and are much better at seeking out friendships than me, because its been necessary for them to do that for their whole lives, if they didn't want to be lonely.
When life gives you lemons, make apple juice. Then sit back and watch the world wonder how you did it.
"Of course it is happening inside your head, but why on earth should that mean that it is not real?" -Dumbledore
And by the way, everything in life is writable about if you have the outgoing guts to do it, and the imagination to improvise.
The worst enemy to creativity is self-doubt.