OMG! I an so sorry to hear this.

Did the vet have any idea what may have been the problem?
Keep an eye on Priscilla, she may get depressed too. She is probably just as confused, if not more, than you. She will be looking for her buddy. Watch that she is eating everyday, and give her extra loves.
I just can't tell you how sad it makes me when someone loses a pet. My thoughts are with you, Priscilla and Gracie.
RIP little Gracie.