It's very odd that they are both having the same issue, bc that makes it sound more like a bug/germ/parasite issue. However...when you say they're 'sisters', do you mean they're actually sisters, from the same Sire/Dam? If yes, then it's possible that allergies are running in the line and that they're both having allergic reactions to the same thing.
If I were you, I'd get to a Vet Derm pretty quick to find out what's going on. They're not cheap, however, you'll likely spend a small fortune if you keep going back to your generalist vet for several frustrating appointments...whereas a Derm can usually figure it out in 1 appt, or at least figure out a plan for you to approach w/ your general vet.
Where are the blisters occurring? Are they also chewing/licking their paws?
__________________ ~ A friend told me I was delusional. I nearly fell off my unicorn. ~
°¨¨¨°ºOº°¨¨¨° Ann | Pfeiffer | Marcel Verdel Purcell | Wylie | Artie °¨¨¨°ºOº°¨¨¨° |