Muzzling is usually counter-productive.
Quite often, a dog bites from fear rather than aggression and removing his means of defense makes him even more insecure and paranoid.
I tried to explain this to a dumb ass with a clearly terrified, muzzled Pit and even showed her how great her dog could be when -properly- handled and she was in awe of what wonders I achieved with him in just 10 minutes.
And then she slapped the muzzle right back on him because the PetCo “dog trainer” said she should.
And the poor dog started shaking and growling again, just like he had been, before.
If I had to bet, I’d say he’s probably dead by now.
The OP’s sister -must- get the dog a blood panel done; or at *least* a full T3/T4 thyroid panel.
90% of Dobes will get thyroid issues at some point in their life.
It’s only a matter of the degree.