The economics of size availability is the problem. Think of a lumber yard that only keeps square lumber (1x1s, 2x2, 4x4s) in stock and tells customers they can laminate two 2x2s with wood glue to get a 2x4 or split a 4x4 in half. Yes, you can, but it would be great if the 2x4s we're kept in stock.
Now multiply the problem across band and cup size. Sister sizes are a way to project a two-dimensional inventory problem into one dimension and that's the root of the issue.
It's not unlike the average cockpit problem[1], but applied to bras. The first person to make an adjustable cup size bra that's comfortable and fashionable will make a fortune.
1. https://www.thestar.com/news/insight/2016/01/16/when-us-air-...
It is video. The handheld portion is shaped roughly like a pen, with a cable attached to a computer & monitor. The lens end is rubbed across your teeth and a 3D image is made.
It had to be rubbed rather firmly against my teeth & gums. Doing so against breasts would be painful. I've had rather severe gynecomastia since I was 11. In my 30s, I'd sometimes wear a bra when doing sports. I wore a 38B/36C (depending on the brand of bra).