Oh! I totally forgot to add my sibling thoughts...
I have one sister who is 2.5 years younger than I am. While I love her, she is also a HUGE stressor in my life. She is very irresponsible and has had to leave the real world and go live with my mom 4 different times (for anywhere from 6months to 2 years each time) in the past 12 years in order to (her words) "get her sh&t together". Of course it NEVER works and she ends up moving out when my mom either breaks down and kicks her out, or my sister decides that Mom is the REAL reason her life is in the crapper and moves out. Blah. Such drama all the freaking time. Anyway, I KNOW the time will come when my mom will NOT be able to be my sisters safety net, at that point my sis is going to come to me and want ME to take her in and be responsible for her and that is NOT something I will do. It is going to be horrible and messy and hurtful and I am NOT looking forward to it.
I did appreciate her most of the time, when I was growing up but ONLY because my parents were so disfunctional that I needed to have someone with whom I could unite against the common enemy. However that being said, we also fought like crazy (violent terrible fights). I wouldn't say we were friends (because I would have never chosen her as a friend and she would never have chosen me) so much as teammates and companions.
My husband has one older brother (2 years older) and they are NOT friends. They don't hate each other or anything, but they certainly are not friends. Calling each other happens maybe twice a year (b-days and x-mas) and they never make active plans to see/visit with each other. Even though his parents moved around a lot (civil service) DH says he and his brother were never really friends.
So, in my personal opinion the decision to have more than one child should be made because it is in the best interests and according to the desires of the parents. Do NOT have a second babe just because you think your first one "needs" a sibling.