Many lingerie stores will try to get you into a sister size if they don't carry your size, or if they have sold out of it. Don't be convinced that this will give you a properly fitting bra. It never will. When I worked retail, we only used sister sizing as a means to determine cup volume, and we would use that information to create a special order if we were out of a certain size. Now, what happens when you are put into a sister size is that the cup will be the best fitting part of your bra. What about the rest of the bra? Your bra band is actually the most supportive element in your bra. I know, it's surprising to think that wearing the correct band size will give you the best support in your bra.
This is not to say that the other elements are not important because they are, but if you wear a "sister size" where the band is too big (this is the case for most women), the band will ride up in the back. If the elastic in your band is not getting worked at all, your band is too big. When the band is too big and rides up in the back, the breasts will actually droop in the front, even with the correct cup size. Then you will find yourself shortening your straps to hold up your breasts, and the back will ride up even more. It's a vicious cycle.
You might think, okay, I'll just go with the sister size that is a band smaller than mine, but you will not be happy here, either. When you wear a sister size where the band size is smaller than you should wear, the band will feel tight. It might actually dig into your skin, creating...dare we say, back fat. And if this weren't enough, when wearing a band that is too snug, it will actually force the underwire that resides closest to the side seam, into your ribs. This is incredibly painful, and for some women, will actually leave bruises.
You deserve a properly fitting bra, and to feel comfortable when wearing one. My hope with this post is that you will think twice when you are being talked into trying a bra in a sister size.