Sisters Attack!

I am new to the forum...glad to find it. I have two female litter mates that are about 18 mos. old. I've had them since they were about 5 mos. I had a problem a number a months ago when they stayed at the vet overnight when they came home and started hissing at each other and wouldn't have anything to do with one another. Before that instance, they were inseparable. Since then, they have become friends again and everything has been great.
I'm not sure what happen tonight, but while I was in the other room one of the cat's (Zoe) got tangled in a plastic shopping bag and freaked out. She dragged the bag that had a few light weight items in it all they across the room under the couch. After I got her untangled, she was hissing and growling like I have never heard her do before. I also found that she had peed on the floor, I guess from being scared. When I found the other cat (Mattie) she was hissing and growling too. They will not go near each other and I have had a terrible time settling them down. They will let me hold them after some initial hissing, but just continuously growl in the other's direction. HELP!!! What do I do? These cat's have been so loving and affectionate and really have always got along (except for the one instance above). Will this last? Any sugesstions would be greatly appreciated!!

We had the same thing recently, too! 2 female littermates, 8 mos old, always getting along. Dec. 23, they were hissing and growling and their fur was all on end, all of the sudden when I got home. They're back to normal, for an unknown reason, now. I hope yours are too!