Thank you delia. Funnily enough my grumpy cat will eat with the kittens but her more chilled out sister won't. As I type this Grumpy Cat and the kittens are eating out of their respective bowls on the kitchen floor, whilst Ms Chilled is eating on top of the fridge (which has always been 'her' place). Grumpy Cat has just viciously attacked her sister again completely out of the blue. She has never attacked the kittens in the same way but we would never leave them alone in the house together, we always shut the kittens in their room when we go out. I've now caught both cats (separately) sleeping on one end of our bed whilst the kittens were asleep on the other end - that's a promising sign, right? And whilst Grumpy Cat is still growling at us she has started to purr when we make a fuss of her again sometimes (she only ever did this sometimes before, she's always been a grumpy bugger! (But she's a cute grumpy bugger so we forgive her!)). She even woke me up in the middle of the night last night butting my hands for me to stroke her and purring. I think she wanted to go out, but still!
There is a bit of luck involved as to whether cats get on with each other, and kittens are viewed by cats as being less of a threat in general.
When I introduced a third cat it initially caused a rift between my two girl cats as they were both on the defensive and started hissing at everything including me. This did wear off though after a few days though I'd say its taken a month for them to revert to normal.
I don't have many tips to help them get along - I wouldn't let very little kittens be unsupervised with the cats. I'm a fan of giving lots of treats to the cats when the kittens are present to create a link between new kittens and reward. It also allows you to see how upset the cats really are - if my cat declines a dreamie I know its very upset. If however its hissing but prepared to eat in the same room as the others I know we're headed in the right direction.
We have two cats of our own, sisters who we've had since they were kittens (they're now just over a year old, and spayed), and we've been fostering a pair of boy kittens for 5 weeks. Neither of our cats was very happy when we first got the kittens, but one has pretty much accepted them whereas the other is still growling at them and runs up and bats them on the head with her paw when she thinks we're not looking. She has also started to growl at us and attack her sister in an aggressive way (snarling, claws out) (as kittens they were always together but they've grown apart as they've gotten older and were generally ignoring each other before this). What can I do to help them all to get along? The kittens will be staying for at least another 7 weeks. We've been trying to give ours lots of treats and fuss so they don't feel left out, even DS (age 4) has been doing this, and they have their own room where the kittens aren't allowed (and vice versa).