Hmmm it sounds like Willow is resource guarding. Harley does this with bones big time. She never lunges at Dilbert but she will growl/bark and then grab the bone. Dilbert won't even be near the bone and she will do this. He'll walk by and then she goes to grab an bone. So It seems to me like Willow might have thought that the bone was there and that is why she reacted that way. When Harley does this I correct her and say no bark. And all is fine. But make sure that if she has a bone, or any toy that really likes, you can always get it out of her mouth without her growling or snapping at you, expecially if you have kids. I have done this with Harley and Dilbert, Harley has never done anything to me, but Dilbert bit me once when he was 3 months old; and we have always tested him since then. He has never bit me or Tim ever since then. Sorry for writing a book here LOL. But by the sounds of it you are right on the money with the bone guarding. I think that they are still trying to work out the pecking order too, and Willow is trying to be on top. Harley is the top dog in our house and she guards the bones too. Hope this helps
Nina