"Sister Sizes" are cups and band sizes that are compatible with one another. This means that, even if we may be out of your typical size, we may be able to use a sister size of an in-stock bra in the style you like!
For example, If you usually wear a 38C and the size is out of stock, trying a 36D may be a better choice than trying a 36C. The cup size is a little large but the band itself is smaller.
BRA CUP EQUIVALENT