Sister sizing is a rough guide that helps you change band size without altering the fit of the cup. The rule is: If you go down in the band, go up in the cup. If you go up in the band, go down in the cup. E.g. If you buy a 34E and you feel like the band is too big, but the cups feel good - we would recommend trying a 32F. That's one size down in the band, but one size up the cup. Exchanging for a 32E here may mean the cup feels too small.