Agreed, add at least 1 more. With a 13 gallon you could add 2 more, that will lessen the aggression further.
It sounds like you haven't done much research on sororities - I'm not saying this to chastise you, but they can be pretty complicated. The aggression is inevitable, which is why you want to have more female bettas to spread out the aggression.
I would recommend reading some existing threads on here about sororities so you can head off problems before they arise - because they probably will, with a sorority. The more you know, the better the environment you can give them.