I have my own opinions as well, sort of a mix of the comments so far here.
I have a cat rescue group, and my own personal cats.
First off, I wouldn't say either sex is a "better cat", but females do tend to be the more bossy of the two.
OP-your 2 females no longer know they are sisters, and the older they get, the more they feel they need to secure their position. Often it will be the smaller/weaker personalitied cat who gets the boot, so to speak.
It's extremely important for you to show the "underdog" that she belongs there too. Be sure to give her tons of attention, not allowing the other cat to interfere, but to watch.
There should also be a time out, where the bossy cat is closed in a room or space so the other cat has time to roam in the house without fear.
When the 2 are together, you should be a referee, and when you see the aggression, make a fast move towards this, say a stern "NO"!, slap your hands together once, even a stream type squirt with a water bottle if you must, then put the bossy cat in a room for a short time out.
This will not only show her this is not acceptable, but gives more power/confidence to your other cat.
They are very intelligent, it won't take long before things balance out, at least to the point of tolerating each other.
This is exactly what I had to do, my now 10 yr old cat Krissy, I got the very day she was born, she is my heart to say the least, and the very reason my eyes opened up to homeless/unloved/unwanted cats and the start of my successful rescue group. Because of the love for Krissy, more than 1,000 cats and kittens have forever homes, instead of killed at the shelter, hit by cars, killed by animals outdoors, etc.
About 4 1/2 yrs ago, I rescued an all white persian cat Simone. She stayed at my store for 3 months until her and I bonded and she wasn't afraid so much.
When I finally took her home, this gorgeous, feminine beauty pounced on Krissy immediately, along with any other cats near our yard, etc. It was clear she was used to fighting for what she needed.
It was so upsetting, my Krissy's secure place in my home was rocked, she became depressed, etc. and it hurt me something terrible. I did consider rehoming her, but Simone needed me too, so I figured this system out, had to do this for some time, but we now live in harmony, Krissy maintains her place even though Simone could clearly kick her you-know-what with no problem.
No, they don't like each other, but tolerate each other, which is all that I care about.
I do have to always stay aware of things like not allowing Simone to sleep in Krissy's spot on the bed, since Krissy will refuse to lay where she smells Simone, or the little control things that pop out occasionally, like one will be around the corner and waiting to jump out on the other, it sort of goes back and forth though.
You can control this behavior, but it does take consistency and some time.
I hear about Feliway, haven't tried this myself but my system didn't have the need for it.
Good luck to you and your kids!
