Cairn sibling rivalry

[Susanjoy]

My 10 month old cairn, Piper is an absolute sweetie.  She’s our 4th cairn terrier.  However, she is extremely aggressive with our 14 1/2 year old female Westie.  Aggressive to the point of the poor older gal needing stitches after her last altercation.  The Westie is completely caught unaware each time.  Piper has been thru training ( still a work in progress).  She is otherwise a great little puppy.  I should add that we also have a 6 year old wheaten cairn mix ( male), who is never part of these battles.  In fact, he runs away.

[bradl]

Others will have different experiences and opinions. Just some random thoughts. We have lived with bitches who were, uh, incompatible to put it mildly.

• Piper is at an adolescent age where it is very common for dogs to begin pushing boundaries and thinking about taking over the joint. Sadly, we also observed particularly with bitches that the presence of an elderly or infirm dog is pretty much nature's invitation to take over.  You have my sympathies during this time. While the boys sometimes squabble it is *usually* more sound and fury, but very little damage. The bitches on the other hand tend to play for keeps. 
• You may be at one of those places where since we can't really control the dog it may be easier to control the environment to prevent even the opportunity for altercations.
• FATS (food, attention, treats, and space) are all things that can be controlled or monitored to see if you can head off a spat before it gets launched in earnest. Making sure they don't end up in a confined space (bathroom, hallway, doorway, etc) together can go a long way to keeping peace. When we sensed a tiff brewing we would sometimes just all go outside and things would typically de-escalate immediately. Winter may make that less of of an option. 
• If circumstances allow, the more activities they can safely do together the better, to build a history of cooperation as a benefit to both of them. One breeder I know who lived in an appropriate setting would take her squabblers hunting together. Going to a group obedience class can also be very positive. In our suburban situation walks around the neighborhood can work as a collective activity. Only you will know if that is possible in your environment— adding tangled leashes into the middle of dog fight, hoo boy, not great. If you have a companion helping so each can walk/manage one dog, that's ideal.
• Speaking of which, you have considerable experience but for anyone who might have less, think in advance about how to separate dogs who are in it to win it without being bitten yourself. We've had the best luck with grabbing two *rear* legs and simply backing away, dragging the first dog we can reach away and out of reach of the other dog — even to the point of spinning around to get the dog in the air a bit like playing "airplane" with a child. Once they are safely out of reach of the other dog you can hold (support) them more normally. A tail can work to lift a Cairn or Westie briefly but I would not spin them that way out of fear of injuring them.  The two rear legs method is safer and less likely to injure yourself or the dog.

All that sounds a bit dire. My guess is that after a while they will sort things out and the old girl will let the rapscallion pretend to be boss of the world. These are just some ideas that can hopefully help getting there easier during what is hopefully a phase.

[Susanjoy]

Thank you. I should have mentioned we are out of town and this incident happened under our grown kids care.  Piper is definitely attached to me and could also be somewhat out of sorts.  We will be meeting with our trainer once we get home.  Any and all information is welcome , helpful and considered.