Bare minimum. I'd rather they be in a 10 gallon. But 5 gallons is the bare minimum, and not abuse. It's like how you can can keep an angelfish in a 20 gallon, but it would be better to have it in 29 and up.

Large male bettas definitely need a 10 gallon, I'll give you that. Females and smaller males will be ok in a 5. Every professional aquarist on earth would agree with me that the bare minimum is 5, and that people who otherwise are elitists. People who like to wave their moral superiority over others.

5gallons isn't really abuse, it's mainly just elitists that say that. 5gallons is perfectly fine, while I'm sure a 10 gallon would be better, 5 is ok. It's why also why I said the bare minimum, and it is in no way "abuse".

Also, sororities almost never work with bettas. It is HIGHLY recommended to keep them alone, and I have heard nothing but horror stories about females getting along for a while and then killing each other out of the blue. If anything, keeping two very territorial and solitary creatures together is abuse in a home setting.

I have no idea what 2m is, but it sounds a lot smaller than a 5 gallon tank which is the bare minimum