Instead, the next size youll want to try is a 38B, which is the sister size that would deliver the right cup volume while also giving you a bit more give in the underband for added room and comfort. At a glance, it may seem bizarre that a woman whos always been an A could ever consider a D, or even a DDD, as a viable alternative.Bra Sister Sizes - How & Why They Work | Esty LingerieJul 08, 2019 · Bra sister sizing is the rule that when you go up a band size, you also need to go down a cup size, and vice versa. Occasionally, you may even want to sister size more than once. Occasionally, you may even want to sister size more than once.Bra Sister Sizes What They are and How to Use Them bra sister sizesWhen you go down in the band fit you need to go up in the cup size to keep the cup volume the same. So, a 10DD is a sister size to a 12D. It works the other way when you go up in the band size, from 12 to 14, while going down in the cup to a C. 14C is the other sister size for