It's not uncommon to want a second child, especially whilst the first one is still young. It doesn't really matter whether one had a close sibling relationship or not.

Did you guys not address the children question before getting married? This way you could have been on the same page and each other's reaction would not come as such a surprise.

I would have a frank discussion with her when you are both calm i.e. you not wanting kids. It's better to be upfront rather than lead her on with 'let's revisit in 6 months' just to placate her. IF she wants more kids and you don't, be honest with her. She deserves to know that.