Cat attacking other cat... After 4 years of living together

OK, here's our situation.
We had three cats that have lived together for over 4 years until recently.
Two boys (Joe and Chester) and one girl (Anna).
Chester was lost to cancer recently, and nothing could be done to save him.
The boys were best of buds... cuddling and grooming each other all the time.

A while after we lost Chester, we decided to adopt a new kitten (Aaron).
Aaron was 5 months old when we got him a few weeks ago, and Anna loves him already.
Joe... On the other hand, was very apprehensive, but started warming up to him being around after a couple of weeks.

Here's the recently developed situation...

Joe has started aggressively attacking Anna... I'm talking, serious aggression: screaming, clawing, chasing, biting... Just a furious amount of hate and violence towards Anna.
This happened the same day that I happened to be playing with both Anna and Joe together with some strings... They were happy the same day it happened, and nothing had changed. It was literally an hour after they were playing together.
We now have to keep Joe downstairs, and Anna and Aaron upstairs.
Any time he even hears Anna meow, or sees her paws under the door, he just charges the door and starts screaming.

I don't get it! What could have caused this?
I thought we were making progress with the new addition, but now we've completely moved backwards and the train has de-railed.
We can't have "upstairs cats" and "downstairs cats". That just won't work...

Maybe it's unrelated to the kitten. Sometimes cats will attack another cat that is sick. They can sense things we can't. Is there a possibility that Anna may be unwell?

Or, it could totally be related to the kitten. Sometimes cats don't recognize each other if their scent has changed. If Anna is spending a lot of time with the new kitten maybe she smells like him. Have you read the posts on new intros? It may be time to back up and pretend none of the cats know each other. Give it a fresh start.

Yeah, I've been doing lots of reading...

I'm afraid we might just have to start completely from scratch.

I didn't mention (to avoid confusion in my story) that about a year ago, Anna's sister (Sidney) passed away from a form of kidney disease. Anna and Sidney were from the same litter, and they look exactly the same except the coloring... Anna is 9 years old now.
And before Sidney got really sick, Joe started attacking Sidney in the same manner.

I hope Anna is not sick, but she acts completely normal, has no jaundice or weight loss... And is eating like a horse.

Since her littermate had that trouble a trip to the vet might be a smart idea. Animals can be much more tuned in to that sort of thing than we can. Maybe catching it early can head it off.

Maybe Joe attacked Anna due to re directed aggression. One of my cats was looking out the window at a ferral cat once, then turned around and went after one of his buds. May be a possibility

You also have two major shifts in pecking order on your hands. The pecking order may have shifted after Chester passed and then you added Aaron, which further calls the rank and also the territory into question. It may be a sorting out of status and turf.

Good luck!