We have three cats: Sprinkles, Lucy and Noisy. Sprinkles is the mother of Lucy and Noisy who are 10 months old. Noisy had 3 kittens a couple of days ago. Since the kittens were born, the Sprinkles and Lucy have become agressive to Noisy. They do not appear interested in the kittens (so I'm not really worried about them hurting the kittens) but I don't understand why they have become agressive to Noisy. Is this normal? Will it pass?
well this is normal as cats are not pack animals like dogs and generally do not like to live in large groups in confined places. territory is very important and even though they are related to each other they do not have the family bonds like us humans.
So now there are 6 cats living on the same territory and as far as your two cats are concerned this is too many.
the aggression is probably stemming from the fact that the mother cats smells of the kittens and they are the strangers in the home.
As to if it will pass, well possibly but not always, generally when too many cats are on the same territory they will try to drive some of the cats away and this could be what will happen with the new mother and her kittens.
I don't know what your plans are for the kittens long term, but I would say that you will have to seriously consider re homing the kittens once they reach three months old.
perhaps you could try and keep the kittens and the mother cat in a separate area to your other cats until this time and then once the kittens are rehomed slowly re introduce the adult female to the other cats again using the process described here
I would also ask you to seriously consider having all your cats spayed and neutered in the future, this will stop any more kittens being born and causing over population and territory issues as well as reducing your cats frustrations etc which can also cause aggression.
best wishes Kate