When Should I "Sister-Size" Bra Fit?

Let's first talk about what sister-sizing means.  It's a term used when you adjust to a smaller or larger band while still having the same fit in the cups.  Cups are based on volume and sister-sizing accommodates that, to an extent.

To illustrate...

A 34C "sisters" to a 32D when you need a smaller band.
A 34C "sisters" to a 36B when you need a larger band.

Image source: https://thelingerielesbian.tumblr.com/post/65660813129/im-feeling-alittle-confused-about-bra-sizesi

This really only works when you're working within the same brand and style.  Mainly because sizes between different brands don't translate exactly.  You can't go from a 34C in Panache to a 36B in Chantelle because they're constructed differently.  But it becomes even more important in the larger cups.  The sizing becomes quite disparate above a DD cup.  Panache uses UK sizing and Chantelle uses EU sizing.  So where a Panache 34F sisters to a 32FF, Chantelle 34F, sisters to a 32G.

Even within the same brand, you can't do a straight sister size from one style to the next because, again, the construction and materials are often different.  A cup with one piece to the cups--think molded cups--are going to fit differently than a cup with multiple pieces.  Always start with the band size that has been determined to be your best fit, check it for snugness (upside down is one good way to do that) then adjust for cup fit.

In some instances, sales associates at well-known stores have taken sister-sizing to the extreme, in an effort to fit the breast volume.  This is completely incorrect.  Always fit the band first.

But, what do I mean "to the extreme"?

Say someone is UK 32G.  In shorthand, sistering looks like this 32G - 34FF - 36F - 38DDD (UK E)

When you consider that the band is what your ribcage measures around its circumference, it becomes quite obvious that someone who measures 38 inches will, in no way, fit into a 32 band.  Conversely, someone who measures into a 32 band will be swimming in a 38.  Where the sales associates are able to "make it work" is when someone has breast volume well beyond any cup that they carry (example: UK HH cup) but they want to make a sale.

Well, if the store sells 38DDD, sales associates have been known to "encourage" their 32G customer into it because it "totally fits".  Yeah, no.

Say it with me: Always fit the band, first, then adjust for the cup.

If the cup sizes don't go up high enough in a particular brand, you're going to have to find a different brand.

This may help with that:

28J+ : Curvy Kate, Ewa Michalak, Comexim, Panache, Bravissimo
30HH+ : Above + Freya, Tutti Rouge, Fantasie, Kris Line, Parfait
32H: Above + Flirtelle, Pour Moi, Shock Absorber, Gorsenia, Samanta
34GG: Above + Elomi, Avocado, All Undone, Miss Mandalay, Ewa Bien
36G: Above + Claudette, Goddess, Gorgeous, Prima Donna, Sculptresse
38FF: Above + Wolf & Whistle, Gossard, Change, Chantelle, Alegro
40F: Above + Cacique, Natori, Wacoal, Fayreform, Curvy Couture
42E: Above + Montelle, Kiabi, La Isla, Addiction, Ava
44DD: Above + Bali, Soma, Wonderbra, The Natural, Best Form

(source: https://www.reddit.com/r/ABraThatFits/)

The main takeaway is this: Always fit the band, always trust your gut on fit, and don't let anyone pressure you into buying what you don't actually love.  Sister-size when the cup fits but the band doesn't quite fit.  If it still doesn't fit, it may be a shape mismatch and you need to try a different style and/or brand.