I do that regularly with sisters. I've also kept unrelated female bettas in dorms (sororities). The key is to provide LOTS of plants for them to hide in and keep your eye on them all the time so you can remove any that become too aggressive. In the beginning and occasionally from time to time they will challenege each other to determine who is top fish (yes, even the females)--usually one will flare like a male does, or they will line themselves up tail to head and do this bumping each other with their tails and/or sides (nothing bad, just this pushing and shoving contest). Sometimes if you've got an exceptionally aggressive female, she'll go about charging and biting and tearing at the others. Occasionally a gang of girls will suddenly decide they don't like one fish and they will pester her (sometimes will nip her) until she goes into hiding (and may die because they won't let her feed or gulp air) or dies from wounds or stress, or then they suddenly decide she's okay and leave her alone again. Basically, just keep your eyes on them so you can step in when things get too rough. I don't keep my breeding fish in any group situation because I don't want them hurt or psychologically too damaged to want to breed.
But then, I kept a tank full of male and female bettas for about 3 years (all siblings) and they were all just fine, a little sibling rivalry every now and then but basically they just lived like a family of sibs do (and now they are all slowly dying off one by one). LOTS OF PLANTS.