These things happen sometimes. Siblings turning on each other.
If you both work, you can't be home to monitor the white and black
cat. You say the white cat is the predominant one, but you also say
it's the aggressor. I think too that the grey cat episode has contributed
to the problem. I know you are cat lovers. I love cats too. I have
taken care of several cats from animal shelters, not all at the same
time. We currently have another cat we rescued two years ago.
I'm sharing this with you because I want you to know I do know some
things about cats. At one time, I had my own cat and I was also looking
after two of my younger sister's cats. It's a long story. Anyway, my
female cat was the dominant one because she figured she arrived first.
My sister's cats were a female and a male, but Mandy, that was my
cat's name was definitely the dominant one even though the male cat
was almost twice her size. I think it was a territory thing. She wasn't
mean to the other two cats or attack them. She just was the boss.
It's unfortunate this happened, I mean the plastic bag incident where
you say the white cat got into the plastic bag and the black cat tried
to catch her ? What that it ? Cats have good memories and some of
them hold grudges. I know a case where a mother cat who had three
kittens turned on one of the kittens and tried to attack it. Squabbles
happen in cat families too. Your problem is an interesting one.
Here's an idea. You could try putting the black cat into the spare room
instead of the white cat being as the white cat is the dominant and
aggressive one. The white cat may feel it's being punished even though
it was the white cat that got tangled up in the plastic bag.
Having the white cat out with you, I would let it settle down, it will be
annoyed from having been cooped up in the spare room. Cats don't
understand punishment or consequences. I read this in one of the
cat reference books I own. If the while cat has become aggressive from
being locked in the spare room, it's time to let the white cat out.
Put the black cat into the spare room instead to keep her safe while
they both calm down. You may be able to restore peace this way.
I wouldn't advise letting the two cats be together while the two of you
are at work. A good friend of mine has four cats. Two of the cats don't
get along and one of the cats has to be separated from the other three
cats and has been for awhile. It's not the most ideal situation, but those
four cats were all rescues. Bottom line. You could try what I've suggested
and see if this improves things. The two sisters may be able to work
this out. Give it some time, These things take time. Also make sure the
black cat doesn't think it's being punished. If you can, one of you could
spend some time with the black cat and one of you can spend some
time with the white cat. Cats are have different personalities, bless their
hearts. I'm a big cat lover too. Peace, Eve
You just never know what is going to incite territorial issues with animals, it can be a new toy, too much discipline, the smell of the vets office on a sibling when he comes home from getting shots or checkups. It can also come from other animals that the pets can see out of windows or in the hallways of apartment buildings.
There are calming sprays and plug ins that you can buy to spray on furniture or blankets or their beds. They aren't the cheapest things to try, but they do work to calm the pets down. I have had my girl spitting and howling and after spraying her fav blanket, she laid down and went to sleep.
There is an excellent show to watch on the Animal Planet called " My Cat From Hell, and the guy from the show deals with problems just like you are posting here. He recommends that every cat have enough exercise during the day to ward off agression and uses a lot of those little bird toys that you can make yourself or buy on the internet.
Good luck to you