Why is my mums dog attack my dog?

Hi there. Both my mum and I have dogs. They are both from the same mother. They are both Collie cross Labs (more collies) They are both 4years old. Recently My mums dog (Buster) has been aggressive. If I take my dog (Bex) out for a walk then bring him back in the house Buster will start to growl and then lunge for Bex, Buster is becoming very vicious and they fight quite aggressively. And now the past couple of days he has been attacking Bex even if I just take him out to the toilet or have him in my room for a few hours then bring him to the living room. Why has this happened? I'm afraid for my dogs safety as Buster is getting very aggressive. And I'm afraid he will turn on family members. Please help asap
   
Nicky from Scotland