Wow, that is really strange, since they're littermates. My sister has littermates(boys) that have been neutered, they have had a few fights, mostly over food, but, for the most part, get along great. The muzzle usually can only be on them for a few minutes at a time.
You have to be very careful as yorkies will fight until one is dead. This has happened more times than you could imagaine.
Have they always been like this? I guess those are 2 sisters that really don't like eachother.