Kittens not friends anymore... HELP

This time last year we rescued 2 little kittens (girls). Their mother had been run over and the kittens were found dumped on the side of the road.
Daisy the bigger kitten has always been very friendly and cuddly and Bluebell was always timid and shy. She is much smaller than Daisy. The have been alowed outside for the last 4 months or so and recently Bluebell has started swiping Daisy. Daisy now cowers when Bluebell comes near and they sleep in different rooms where intitially they were inseperable.
They were both nutered at 6 months. We have another very large 6 year old male cat who only tolerated both of the kittens and dishes out the odd serve to them.
Does anyone have any advice regarding the kittens as it seems that they are destined to hate each other and we are getting worried.

A visit to the vet to make sure it is not a health problem. Cats can sense when one is sick and they tend to avoid the weak one.
If all is okay, it could just be that Bluebell is now the dominant one. Bullying her sister is her way of reminding Daisy she is the boss.

One issue that comes to mind is have they both been spayed? If they haven't been fixed, they both should be as their behavior may be reflecting hormonal maturation and territorial issues.

Letting them outside could create anxiety issues. Sure, they love it, but there's a lot of scary things out there. This can cause displacement of the fear and aggression. If Bluebell was frightened by another cat out there she could be trying to make herself feel better by swiping at Daisy, who is less frightening to her.

When we let cats out we have far less control over what they encounter and how it will affect them. If you want to cure it you need to keep them in where you can monitor their encounters and do things that let them associate each other with good things, like treats and playtime together.

You can heal this relationship. But it's much harder when you have no control over what they are encountering out there.