H.D.
A.,
I was an only child until I was 12yrs old and then within 15 months I had two sisters. I liked being an only at times growing up, but most often I was jealous of the neighbors who had 4 siblings. There was always someone for them to play with, they were never alone (though maybe they wanted to be at times) and they seemed to take more "kid friendly" vacations instead of adult oriented. Once my sisters came along I couldn't imagine how I did it without them. They mean so much to me and now they are so close to my own children.
I also agree with another responder who said (thinking way into the future) that it would be hard to be alone at the wake/funeral service of our parents.
I say let nature take it's course and if it is meant to be, it will. If your Daughter is an only child, it is certainly not the worse thing. :-)
Try not to think to much about another child but enjoy the one you currently have. Time goes by much to quickly.. My "baby" is almost 10 and my oldest is 18 with two others in the middle ...