sibling fighting

Question
I have two female/spayed 3 yr Lab/Weimaraner who have just reached social maturing.  They had a terrible fight that resulted in both going to the emergency room.  I cannot get them together without Izzy attacking Aly.  Aly has immune deficiency syndrome and was diagnosed with Addison's disease last October.  Izzy will stare at Aly and intimidate her.  Aly is scared of Izzy and it shows.  These two dogs lived together in the same area and slept together.  To keep each dog safe we had to put purchase kennel fence and keep them separate.  They are walked separately.  Izzy cannot be taken off her leash because she will approach Aly and attack her.  Izzy stares at Aly when they are in their fenced in area.  Everything was fine up until this fight.  I had a behaviorist come in and was told that we would never be able to get the dogs together again.  Izzy blames Aly for being in this new "environment".  If Izzy would leave Aly alone they would both be able to be let out in the evenings to pay together.  My options keep pointing to having to get rid of a dog.  Why does Izzy now want to constantly attack Aly?

Answer
Two females who fight on sight are almost impossible to reconcile.  It's quite possible that the healthier of the two scents (and visually ascertains) a problem in the other, and that might be provoking the situation.  But honestly, it makes no difference.  The behaviorist you consulted is correct.  It's highly unlikely that the relationship between these two will ever be corrected.  I suggest you rehome the healthiest of the two, as the other is simply not rehomable due to her health problems.  In future, never acquire two puppies from the same litter...NEVER.  It often leads to problems (especially if they are same sex.)  And if you must acquire two dogs at the same age and the same time, get opposite sex (and be sure to spay and neuter.)