I grew up for most of my childhood years as an only child. I was 12 when my half brother was born and the following year my mother married my step-father and I gained a step-sister who was 9 at the time. My half brother lived with his parents (my father and step-mother). My step-sister moved in with us after the wedding.
The transition to the "new family" didn't go very well. My step-sister was angry and bitter over so many things - and rightfully so. 9 is a difficult age to try and understand why your mother suddenly doesn't want you living with her and your father found a new wife. I certainly don't fault her, or anyone, at what a mess it became.
I am about 2.5 months from throwing my 8.5 year old son into the closest situation he will ever have to what I went through during those years. While his parenting situation has not changed - his father and I are still VERY happily married after nearly 10 years - his life is about to be turned upside down. He's going to become a big brother.
I'm scared for him. I'm scared for me. I'm scared it will end up as BIG a mess as my brother and sister did. I'm scared he will, for the first time in his life, feel unloved or unwanted. I'm scared he will sacrifice so much for those he loves that he will lose himself. I'm scared I can't be a good enough parent to two children - especially after having just one for so many years.
I'm scared to death. I don't have a "good" model to follow of loving two children. In fact, the vast majority of people I have known in person in my life did the very best they could, but I have nightmares of sibling rivalry including a few who actually tried to murder their infant siblings.
Mostly I'm scared my son, who is one of the most loving and kind people I know, will go the opposite way and be so in love with his little sibling that he will stop doing the things he enjoys and wants to participate in so he can help with the baby more. He's already trying to give nearly everything he owns to the baby (we keep telling him no and that it's not necessary and we'll take him to buy NEW things for him to give the baby).
Today or tomorrow I'm taking him to go and purchase a stuffed puppy for the baby. The nursery is being done in 101 Dalmatians. He wanted to give the baby his very first stuffed puppy (a dalmatian) as a gift and I told him we'd go buy one for him to give the baby instead. I don't think I could keep my heart together if he did that - it would burst from love. He carried that puppy from the moment he got it (his first Christmas) through my husband's deployment and injuries. It went everywhere with him until he started preschool. That's something special he needs to keep for himself - he may need it again in the days to come...or I might need it to catch my own tears. :)
Mental note - buy LOTS of kleenex for the nursery. :)