Cat problem

Don`t know much about cats SP but perhaps you could use positive reinforcement. Give her loads of cuddles or a treat if she behaves well any time with the other cats , ignore her bad behaviour while giving the attention to the other cats . ( It workd with kids

I've heard of that spray sweetpea, Feliway, it's supposed to make them feel calm & confident.
I expect Millie is feeling put out at the 'intruders' in her home, I keep reading that although people think cats are happy in a 'pack' like dogs are they much prefer to be alone, I know our last cat Amy never wanted anything to do with other cats who wanted to play in the garden with her etc. A vet was answering a similar question in the paper the other week where a family had taken in a stray & their other cat hates it, he suggested trying the Feliway spray & if possible giving each cat a room where the other wasn't allowed & as a last resort he said reluctantly find the stray a good new home without a resident cat.

I don't think I can add much here that hasn't already been said. It has however taken my male cat Gandalf about eight months to finally accept the sprightly young female cat (Poppy) into "his" home. They usually get there in the end even if it's just toleration and nothing else.

I fuss each of mine in equal measure separately and always make sure if I do it to one in sight of the other, I always break from it and fuss the other (sooner rather than later).