My family has just gotten two young female chihuahuas. I am new to owning dogs but my partner is not. He has been able to train the dogs to do basic commands like sit and roll over. However, one of them is particularly aggressive towards her sister and towards us. She will attack his sister out of nowhere (she is chewing her bone and her sister chewing another in the corner) even when her sister is nowhere near her. She will attack us when we try to pick her up to put her and her sister in the crate at night and she doesn’t want to go. This isn’t regular puppy nipping from what I can tell. She doesn’t appear to be playing as she has drawn blood from us and the other dog multiple times. Her sister is not aggressive at all but does not appear to submit to her either. How can I teach the aggressive dog to stop attacking and biting when she is upset? I’ve read mixed advice in the forums about how some training methods are outdated or disproven so I’m unsure.