You can see that size 40B, 38C, 36D, 34DD, 32E, 30F & 28FF all use a size 40 underwire. They are all sister sizes- sizes that have the same cup, but different band lengths. That means that you can take any one of these sizes, and create one of its sister sizes by modifying the band length of the pattern.Find Your Bra Sister Size With Our Simple Chart bra sister sizes10 rows · Jan 09, 2017 · For those of you wondering, the cup volume of a sister size is closest in fit to your true bra bra sister sizes 30AA 30A 30B 30C 30D - 32AA 32A 32B 32C -- 34AA 34A 34B --- 36AA 36A See all 10 rows on blog.parfaitlingerie bra sister sizesEverything You Need to Know About Bra Sister Sizes - The bra sister sizesMar 01, 2019 · Sister sizes are alternate bra sizes where the cup volume remains the same regardless of the band size and cup letter. For example, a 30F and a 38C are technically the same size because the cup volume is the same. Someone who is a 38DD has more breast volume than someone that is a 30DD.