I wouldn't worry too much about them being sisters as long as the tank is heavily planted. They will stress out at first but as long as they CAN hide from one another, they usually won't bother. I now have 13 girls in a 29 gallon and they basically act like schooling fish. My recommendation is lots of floating plants that have roots that extend down, so the girls can hide from each other near the surface and get air or like OFL lots of tall plants. Caves are a little useless as long-term hiding places for a betta because they have to come up for air, so if they're being harassed a cave isn't a good place to hide, but floating plants are.