I have two female Wire Haired Terrier and Chihuahua mix sisters. They are two years old and recently they were both fixed. One of the sisters has become agressive to her sister. How do I stop this behavior? Ive used the spray bottle of water and it deterred the fight but if I blink for a second one of them attacks the other and bite each other. What can I do? Please help! I love both of them very much and don't want to give one away. Thank you for any advice.