What is "sister sizing"?
Zookie, honestly, you don't deserve your badge for being a boob man if you do not know what sister sizing is!
Brassiere measurementScroll down to the part about "cup volume relative to band size". It explains the concept of "sister-sizing", but not very accurately in my opinion. Of course, this being Wikipedia, YMMV, so take what you read with a grain of salt. As long as women are skittish about their bodies, there will be no true and definitive way of determining bra size, so women, grow up, get a pair (literally), and be proud of your boobs!

Q-"
Preaching to the choir"-BE