I agree; you are going to have to take her alone to see her reaction. She could be protective of her sister; but she could be just growing up too and feeling her oats as they say.
This is something you can work on if it continues. Ringo was horrible when other dogs would approach us while on a walk. I used the distraction and 'watch me' techniques to take his focus off the approaching dog.
Victoria Stilwell has some good shows on this.
__________________  Mommy to Lucy, Ringo, and Matthew |