Unfortunately, sometimes bunnies, siblings or not, do dislike each other, and as they have fought badly enough to cause injury, it is possible you will never rebond them. I have two rescue girls, who I think are sisters, and they would fight if they got the chance, and they have to be kept apart at all times. It is as much about personality as gender, as i used to have a pair of happily bonded boy buns, and the two boys who are each bonded to one of the girls would get on fine together if the girls would let them.

I hope you dont mind a little story

My nethie, Bombur, turned from a cuddle baby into a huge aggressive jerk during his puberty and attacked his bonded mate, Blue the mini rex. The two had their own cages, but they were side by side. They would always be let out together for a few hours to free roam the living room. They would explore, play, get into trouble, eat, sleep, and cuddle together while they were out. There would be nipping now and then, but nothing bad or overly aggressive, just “youre in my way, *nip*” kind of nips. When Bombur hit puberty, he started to bite instead of nip, and he bit hard! He started with attacking me every time i entered his cage, or if he didnt like where i was sitting/standing in the living room. He saw those as his territory. Blue is my ESA so he doesnt like when i get hurt, it caused them a bit of tension, but they still acted normal together so i didnt separate them during their free roam times yet. It wasnt too long after he began to mount Blue to prove dominance. Blue really didnt care too much, hes much bigger than Bombur so he just shook him off. Bombur didnt seem to mind. Until one day, when Blue shook him off and he attacked full force on Blue. Blue can hold his own, and Bombur lost the fight. No major wounds, just a few spots of pulled fur. I was able to split them a few seconds after it started, but i have no doubt it would have ended terribly had i not been there. They were completely separated for about 3 months. No seeing eachother, only smelling eachother due to being in the same room. Bombur was completely blocked off from the sides as he could claw and bite through the bars, and would lunge and hurt himself. After the few months break from one another, and Bombur getting healed from finally being neutered, i put them in a box together. Blue nipped him a few times, i shook the box, he stopped. Since they were previously bonded, it went very well bonding them again. Blue never held a grudge, thankfully. After a few days of Blue mounting Bombur and Bombur grooming Blue, they were solid again. Now theyre together 24/7 as they share a cage and share free roam time

TLDR
Some rabbits get very, very aggressive from their hormones. I have scares up my arms and a few on my lower legs from my aggressive puberty bunny. He caused a bad fight between him and his bonded mate, but after a few months away from eachother, and after a successful neuter, they were back to being bonded. He is still a little mean to people, but he no longer leaves scares from clawing/biting/boxing. It will take time for them to be apart to cool off, forget their grudges, and being healed from their neuters. Dont lose hope yet

Is it your vet who wants to wait until the rabbits are 6 months old before they are neutered? Some vets do prefer to wait until the rabbits are a little older, but others will perform the neuter surgery as soon as the testicles have descended. Maybe you could speak with your vet and see if the surgery could be performed a little sooner.

Rabbits can fight and injure each other through the bars of a cage, so you might have to use an x-pen or sections of wire cube grids to surround the cage of the rabbit who is caged at the time, so that they can’t physically reach each other. If you don’t have either of those, try to block off their access as much as possible and be very careful when one is out for exercise. You don’t want any severe injuries and you don’t want the rabbits to come to hate each other and not be able to bond after their surgeries. X-pens are good to use for bonding, since you can get in there with the rabbits, so you could use one now to block off access and use it later to hopefully bond your rabbits.

Im glad it inspired you Im sure your pair will be able to bond later on after a good break. A break does wonder for angry buns

Yep! Keep them completely separated until they are both neutered and healed (atleast a month after the surgery). Its rarely possible to bond hormonal rabbits, so its better to be safe rather than sorry