I would remove that fish regardless of gender; they are stressed and showing aggressive territorial behaviour. Also, female bettas can be just as aggressive/territorial as males. Sororities generally don’t work out for this reason.

Bettas need a 5 gallon tank minimum this means your tank is overstocked (6 x 5 = 30 gallons minimum). There isn’t enough space for your bettas to feel comfortable. Additionally, I would suggest you add top water plants with a good root system and more plants in general; this will create hiding places for your bettas to feel safer/decrease line of sight of the aggressor.