Itís really difficult to give specific advice to posts like this, as our subjective opinions as to what is mild bickering versus aggression can be very different.
Yes, Roxo is the right age to be a stroppy teenager having to put up with his Velcro little sister who doesnít appreciate his need for some personal space.
He may view her differently in a few months!
It reads to me, like Roxo sees the cover, knows what is about to happen and wants that sleeping spot for himself. He may have been happy to snuggle up together in the past, but now he isnít.
It seems a drastic step to remove him into a different cage, but I understand your concerns (If this is also occurring during the day and Blueberry is finding it difficult to eat, or if the pecking is escalating into something more or actually causing injury, then they may need to be permanently separated - but it doesnít sound as though this is the case here.)
If I were you, Iíd review the whole cage set up, have a reshuffle around and buy two single swings, in the hope that they will adopt these as their new sleeping spots.
With a change in set up, there may well be a bit of squabbling and grumbling as they jockey for the best spot, so perhaps just wait around with them covered for a few minutes to let them work it out and settle down.
Best of luck.