I have two female,sister, yorkies. They were the best of friends for their first 3 years of life with us. Around their 3rd birthday they began to visciously fight with each other. We are not sure why. A few months after the fighting began we bought Buddy, a male yorkie. The fighting still continues between the sisters and we still have to separate them. We bred our male yorkie with Lily, one of our female yorkies, and are hoping to find out that she is pregnant. Today I bought two muzzles for the females and took them on a walk together. I had to have one on each side of me on the leashes. My goal is to get them to be side by side walking, without fighting. They are only this way with each other. Otherwise they are very calm, submissive dogs. I have a feeling that the fighting may be some sort of a dominance thing going on? I know there is something that I can do to make this stop, but I don't know what to do.