[yorkietalkjilly]

I forgot to mention that the reason and thinking behind crating your aggressive girl while you play with the other one is to slowly, over time allow the aggressor to come to learn that the other dog has your permission to play and interact and do whatever she wants without any input from the wannabe pack leader at all. Seeing the other dog playing and doing all kinds of things while the attacker is safely caged will desensitize her to the other dog's antics, even those she might disagree with and in the past possibly attacked over. The treats will hasten the process and associate all of the other dog's activities and actions with getting great treats and help her to come to grips with these sessions much faster.

Keep her crated until she's able to lie down and just watching the two of you play and is all the while getting her treats throughout the play sessions without focusing in on or reacting to anything the two of you are doing - just her treats. It is not her place to have any opinion about what you two or the other dog is up to as long as you are pack leader and she will come around to accept that you call all of the shots, make all of the decisions and carry out any discipline and love that arrangement as it works well for her, too and takes all the pressure off.

Once she is quiet, calm and accepting of the play sessions in her crate, you can then work with her outside the crate and on the leash a ways across the room from you and her sister and then ever closer, then, eventually, she can play, too, once she's lost her attitude and wants to be just another pack member with sis.