I'll be in the minority with Kirsten
I'm an only child, never had a problem with it. However my Mom became terminally ill and I took care of her all by myself (parents divorced)for 7 months while I was pregnant (She died 3 weeks before my baby came). Worse, I lived in Colorado at the time, she in Michigan so I was away from my husband for our first pregnancy. I would do it again in a heartbeat, but if I had a sibling, there would have been a chance that I'd have had some support or help from someone else. I know I might have had a sibling that wouldn't have made it better, maybe worse like the other poster, but at least there would have been a chance. She needed very personal care that we couldn't ask jusk neighbors or extended family to help with.
Even now, there is noone to be sad with. My dh is awesome, but you can't explain to someone how it feels to not have a mom. Sometimes it would be nice to reminise (sp?) but he hardly knew her. I wish I had a sibling to ask "What would mom do, or did she give you that recipe?"
My best friends parents died in the same week, separate circumstances, when she was 16. Luckily she was the youngest of 5 and her siblings took care of her.
My dh comes from a family of 5, they bicker alot, but all would do anything for each other. His brothers are his best friends no question.
I think this is obviously a personal decision, and all of us have had our experiences. My point in sharing this is just to give a different perspective.
My dh and I planned on one child initially but I have 2, almost 3 children, for the reasons above. If you are a committed parent, as all of you ladies on MDC are, there is always enough love and attention to go around. We have a stable loving marriage and I have every confidence that we'll raise our family to be loving siblings to each other. JMHO.
Good luck in your decision!