Autumn - I haven't actually asked her outright why she wants a sibling, but perhaps next time she brings it up, I will. I have a feeling it has to do with having someone to play with more.
I was sort of raised as an only as well. My next closest sibling is 8 years older than me, and when I was 12, we moved overseas and she didn't come with us.
Laura - I feel the same. If I could go back, I'd likely give her a sibling closer to her age. But that choice was taken away from me (not by my SO, I might add, but by Sophie's Dad, who decided unilaterally that we weren't having any more children).