SISTER BRA SIZES. You may or may not have heard of this, but the sizes that surround your current bra size are called your sister bra sizes. Its good to know what they are and how they differ from your correct bra size. This way, when you try on a bra that is not running true to size, you can quickly figure out the bra size that will probably fit you.What is the size of a bra?Bra is as large as 152.9 square milesSee all results for this questionWhat is a Bra Sister Size? La Bella Coppia Large Cup bra sister sizesJan 15, 2015 · The other sister size will go UP a band size and DOWN a cup size from your bra size. For example, if you wear a a 36 C, then your sister sizes are 34 D (down a band size and up a cup size) and a 38 B (up a band size and down a cup size.) The reason why people think this works is because all those three sizes have the same cup volume.