Much like swapping clothes with your sisters, a sister-size bra has all the necessary functions with a different size label. In a bra, the band and cup sizes work symbiotically. The cup volume is an essential factor in a well-fitting bra. Sister sizes are plural for a reason. Each bra size can have multiple sister sizes, although our recommendation is to stick with the two closest sizes. 

Repeat after me. If you size up the band, the cup size will go down. If you size down the band, the cup size will go up. (Have you caught on to what’s going on here?) The bra and band work inversely when calculating Sister Sizes.