Jun 25, 2019 · Sister sizes are the alternate bra size with equivalent cup volume but different band size and cup letter. For instance, a bra of 38C and 30F are technically the same size as their cup volume are the same. One needs to understand that cup size and band size are not independent entities, but they are more of interrelated one.Bra Sister Size - Size ChartBut what is very interesting, is that the 38B bra actually have the same cup size as the 36C from our last example (!) And the 36B also have the same cup volume as the 34C bra from our previous example. In short: both 38B and 36C can fit the same bust - they are "sister sizes" with the same cup volume. 36B and 34C are also "sister sizes". In short: More bra sizes can fit your bust!Bra Finder - Bali BrasEvery bra size has sister sizes. This means that bras can have equal cup capacity even though they may have different band or cup sizes. The rule of thumb is to go up a size in the band number and down a cup size OR down a band size and up a cup size. For example, the sister sizes