If you can wear a 34B, your sister size would be either 32C or 36A.

Your cup size isn't actually a volume size of the size of your breast. It's a ratio between the measurement of your rib cage (your band size) and the measurement of your bust size. So when you go to a larger band size, you're creating a smaller difference between that and your bust size, so your cup size becomes smaller.

Does that make sense? I sell lingerie and am trying to explain it as best as I can. :)