Good Morning and welcome. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry to hear about Gwen's condition. It could be partly genetics and/or her previous environment that has caused this bad behavior but this will be very difficult to get under control.
As I see it, from having dealt with many of tehse cases, you have 2 options. One is to take this dog to a trainer away from your environment, have Gwen seen by your vet to be sure there is not a medical cause for this behavior, and then have them prescribe an anti anxiety medication used for this such as Reconcile, which is dog Prozac. This will help the trainer to better aid Gwen in reducing her anxiety so this behavior can be turned into a loving dog able to integrate well into your environment. This may not be successful , so you have to be prepared that there is a chance that Gwen may still not be a suitable companion for you and your son and your Yorki.
the other alternative is to return Gwyn...I say this because it appears that she may require a household without children or other dogs....she may be able to integrate better in this environment.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, XXXXX XXXXX