aggression between cats sharing a household

Question
Dear Norman,
I have 4 cats living indoor & outdoor: 2 brothers,Moby & Ringo (6 years) and 2
sisters Mia & Fanny (1 year) and they all get along very well. A few weeks ago
I found a female kitten (1 month) abandoned outside and I took her home. As
I did with the 2 girls, I isolated the kitten in a room with litter, food and bed
for about a week and also used pheromones in many rooms. It's more than a
month now and the 2 sisters are still very aggressive with the kitten,
especially Fanny who attacks her badly. Fanny even started to urinate inside
the house. Though I play with them, pet them all and keep their food and
litter separate, it's not getting any better. What shall I do? Thank you for your
help.

Answer
Sonia,

There is nothing you can do.  Cats maintain their inter-relationships, and how they do it is a mystery to us.

One thing to remember is that cats play very roughly and people often mistake rough play for fighting.  I expect that the kitten, being a kitten, loves to pester the sisters and they retaliate.  As long as no one draws blood, I would ignore any hissing, spitting, posturing, even loose fur.  The worst thing you can do is separate them and interefere in any way with their "discussions".

I would leave them alone and just let them work out how they get along.  Be careful not to attribute human motives and emotions to what is going on as cats do not think like we do.

The urinating outside the house is bothersome and you might make sure you have at least 3 litter pans scattered throughout the sousehold for the 5 cats.  Our rule of thumb is a litter pan for every two cats and one extra for an odd number of cats.

Best regards... Norm.