[AprilLove]

Hi and welcome to YT.
Sorry to hear about your new baby. Has your vet referred you to a cardiologist to confirm his suspicion? If not, you should take her to a specialist in cardiology, as I know from experience, you need more than one person just telling you your baby has a condition by listening with a stethescope. I am a little confused though, if your breeder doesn't know about this yet, how did you know that the sibling in the litter has a murmer?
Did the breeder know this and tell you before you bought a pup from them?
I might not be understanding your post, but am not sure why your vet is keeping your Mia when she should be seen and tested for this by a cardiologist, not a general vet, imo.
Congratulations on your new Biewer, they are wonderful little furbutts!
Will send wishes that a visit to a cardiologist gives you more concrete information/answers and that your new little girl is going to be ok