I've seen this behavior plenty of times.  Something sets them off.  We can
all be sitting out on the deck in wonderful peace and harmony when along
comes a strange cat on the other side of the wooden slat fence.  My cats go
berserk and start hissing, growling, snarling and attacking each other.
Their tails get all fluffed out.  They can take hours to calm down.

When I had a Siamese she would go crazy if I came home with the scent of
another cat on the bottom of my pants.

They should be back to normal in a day or so.

Barb
Of course I don't look busy,
I did it right the first time.

Cats appear to me to be like some humans in that when something bad
happens, they sometimes want to blame someone.  Once when the Vicious
Bag Monster chased Benjamin (i.e. he got his head through the handle of
a plastic grocery bag), Benjie and Caley decided that it was the fault
of Sebbie, cat #3 and frequent Odd Man Out.  They ostracized him to the
basement and wouldn't let him up for the better part of the week.  I had
to set up a litter box, food, and water for him down there because he
was a meek obedient #3 and wouldn't come up until they said he could.

Sounds like both kittens got scared and decided they needed someone to
blame, and that someone would be their sister.

They'll come out of it, but it's a good idea to monitor their
interactions for a bit until you're sure they've come back to normal.

Priscilla

It's called misplaced aggression. They got scared and because they don't
really know at what it is at each other. Keep them separated. Consider
getting a Feliway diffuser for both rooms the cats are in to help destress.
Give them a gradual reintroduction. Hopefully in a few days they will
recover from it.

Give the cats time- however long it takes. Sometimes it can take weeks
for the relationship between two cats to be rehabilitated.
I just dealt with such a situation today which might be worth posting
here.

I got a frantic call from a woman who was referred to me by her
veterinarian.
She's petsitting for two cats that belong to her son's live-in
girlfriend. I'll call them OC for the orange cat and TC for the tabby.
They've just moved into a new house and the cats are isolated to one
large bedroom while the son and girlfriend are gone.

She went to open a window for them and then realized there wasn't a
screen. She closed it quickly but not before OC hightailed it to the
window and got her paw caught just as she cranked the window shut. She
didn't get hurt but it scared her and she immediately went after TC and
then attacked the woman who suffered a shredded ankle as a result. That
was Sunday and the woman had not been in the room since then since she
was scared out of her mind and afraid to go in, and with the holiday she
couldn't get in touch with the cats vet until today and didn't know what
to do. The cats had a big bowl of food but little to no water so
something had to be done ASAP.

I agreed to assess the situation and met her at the house. As soon as we
walked in the door the OC started screaming even though she was upstairs
locked in the bedroom. I peeked in and slid a plate of canned food in
front of me as a distraction and, although it worked, she was
growling/screaming the whole time she was eating. I could see TC in the
background hunched down with wide eyes and hissing.

I ended up getting a carrier and a towel, donning welding gloves and
entered the room. I kept the carrier in front of me to block her access
to my ankles ;-), held the towel up to keep shifting her focus from me,
and was able to slowly back her into a corner. She came at me screaming
several times and bit my hands but it was no big deal due to the gloves
I had on. I barely felt it.

Once I got her cornered I managed to get the towel over her and stuffed
her into the carrier. Fortunately the other cat ran in the closet and
hid. Once I got her secured we separated the two and set OC up in a
bathroom I had already furnished with a large bowl of food, a large
container of water, litter, a pillow for her to sleep on, a radio
playing classical music and a Feliway diffuser. She was still screaming
when I left. TC, however, immediately settled down once OC was out of
the room and, while still hesitant and a little hissy, was visibly
relieved and much more congenial and came out for a few pets and ear
rubs.

I told the woman to leave OC completely alone for a day and then peek in
on Thursday to see what's going on. I set the litterbox up right by the
door and showed her how to drag it out quickly so she can at least scoop
it and also gave her canned food to slide in on a plate everyday to try
to establish a positive association. If the cat is still out of control
by the weekend I'll come out again and we'll see about getting her to
the vet for an exam of her paw just in case, and possible drug therapy
if she's still psychotic.

I think one of the reasons this situation is so off kilter is because
these cats have been stuck in a room together for several days and
probably have had ongoing fights, which just escalated things to a
severe extent. This is why I always underline the extreme importance of
separating cats IMMEDIATELY when a situation like the one you had or
that OC and TC had occurs, giving them ample separation time and then
slowly reintroducing the cats to each other.

It's important to take a lot of time and care when dealing with this
sort of situation because rushing things can inhibit or even destroy the
chances of the two cats becoming friends again. So take your time.

Megn