My younger sister got a new dog on Sunday.
Well now that is behavior I do not tolerate and which happens with Buzz & Emmy a lot. Buzz starts attacking Emmy and then blood flies. What I do is completely ignore both dogs when I come home. I do not talk to them, I do not look at them, I do not acknowledge them in any way for a few minutes and then I go outside and they follow me and then I will calmly greet each one and give each one a Milkbone. The key is to be very calm, keep my tone of voice very low and calm and the excitment level very calm. Have your sister try that with her dogs. My problem is that Buzz & Emmy get into it when Emmy goes beserk if someone rings the door bell. I can't control that behavior. But I sure can control how they react to me when I get home!
Oh and congratulations to your sister and see if she can keep her dogs from running out into the streets.....
The dogs are working out their "pecking order" and competing for Alpha status.