My cat got out of our house and we could not get her. We chased her for an hour around the house and could not catch her. We gave up and went inside. Later that night the door was open and we hear a loud terrifying screech. We went outside and the cat was sitting there just hissing. She ran up a tree when we went to grab her and then she ran into the house. Her sister ( the other cat) was in there and went over to see the traumatized cat. The trauma cat attacked her sister and we had to pull them apart. Now we have to keep them separate because if we put them together they fight to the death. What is wrong? I don't know how to fix this? Is she going to be traumatized for the rest of her life? Please help me these cats are like my kids and it would kill me to have to put one down.

If the one that got out is the aggressor with the other cat, see if it has any bite wounds or if it is damaged in any way and then let it cool off by itself for awhile. I had a former cat that used to get into fights and then was really grumpy and antisocial for a day or two, particularly if he had a bite/claw wound.