I have not been through having a surprise baby, but I have been through the fear and disappointment of having a second son. The great thing is that you already know you will love him/her. And certainly, you will. While I don't have any words of wisdom for you, I can tell you that I am so glad that my prayers weren't answered, and that I had a 2nd boy! I couldn't imagine my life without Connor! And seeing the love, adoration, and interaction between Caleb and Connor is the highlight of my every day. My mom and I were actually talking about this last night. I'm an only child, so she never experienced being able to raise siblings. She's as amazed as I am that we can love two little boys so fiercely and that they love one another with that same intensity. While I can't say whether or not you will feel disappointment if you hear "It's a boy!" at birth, I can tell that if it is you will fall in love all over again, and your boys will have one another for the rest of their lives. And you will be so glad and so blessed you gave them the gift of brotherhood.
Good luck in the coming weeks!