Ok, I went back and found the thread about her arrival. She looks bigger in your sig! Borders and other herding breeds can be nippy. It makes sense for them in many ways--they have to nip at the heels whatever they are herding sometimes. Is it possible that she was trying to herd your visitors? Has her thyroid ever been checked? Low thyroid levels can lead to aggressive behavior. Could she be in pain? My Goldie is a hag when her hips hurt--took us a long time to figure out that she was in pain. Once we did and got her some medication, she was a different dog (even now with her current injuries, she not nearly as snarly as she used to be!)

I'd start with a complete vet check--blood work, the whole nine yards. And then get a good trainer in to help you.

Also if she is BC mix, she needs a job. Working/herding dogs are nothing but trouble when they are bored. Ask me about the ways huskies will amuse themselves! They need a job to do, even that job is fetching a ball 2000 times a day! A tired dog is a good dog.