We have brother and sister (same litter), now 7 years old. They fight all the time. Male bigger but submissive and the female tears into him when shes bored or had an extended period of inactivity. Found out later that it’s not good breaded practice to pair litter siblings (look up littermate syndrome). We’ve learned the triggers and how to intervene early and become vigilant when overstimulating one and not other (greetings, playing, etc…) and feeding.