I wouldn't leave them alone together anymore. Since its happened twice it can happen a third time and the little dog does not stand a chance. I don't know how you can train it out of her, I'm guessing that its because the two females are not far enough apart in age.
Misty- Samoyed Mix
Tannor- Golden Retriever CGC
Robyn- German Shepherd CGC
Cats-Thunder, Harley, Miley, Bandit, and Ferah
RIP Boo..Black Lab
"A dog is the only thing on this earth that loves you
more than he loves himself."