When I bred plakats from Thailand, I gave the sibling group to a man with a 65 gallon open topped tank with large Pistia on the surface, and the roots hanging down. He astonished me as he kept the whole group, males and females together, and the tank had run peacefully for over a year before I lost touch with him.
So a sibling group of females should work, but in a large, shallow tank in a large group. One of the problems you have buying Bettas for such a set up is a lot of farms sell plakat, short finned females to screw up 'compeitive' hobbyist breeding of long finned fancy Bettas. The young never look like the fancy parents from those spawnings.
But in many cases, short finned males get mixed in with the shipping, and that's where 'sororities' can blow up quickly. One local store owner told me that his last ten imports of females from Thailand, Indonesia and Singapore had included anywhere from 10 to 25% cryptic males. He would sort them out by behaviour.