To answer the question from this topic’s name…. yes. This is typical siblings, which can also mean they can be enemies. Rabbits do not acknowledge family units like we do– they will mate and fight with their siblings, parents, and children all the same. The reason rabbits get along with each other when they are younger is much more due to the lack of hormones; not because they are siblings. Now that both of your rabbits have gone through puberty and their ‘baby bond’ has dissolved, it’s time to start some formal bonding steps to provide a safe way for them to establish their life-long bond dynamic.
Firstly, I would wait at least 2 weeks to a month after both rabbits’ neuters before starting. This is because there is a bit of a hormonal “craze” that can happen directly after a neuter which takes time to dissipate. In some cases, it seems to last longer, but typically that’s apparent behavior wise (i.e. their hormonal-driven behavior remains amplified and frequent). It will dissipate eventually.
Secondly, if there is any risk of one rabbit injuring the other, they should be separated when you cannot supervise. Regardless if they were able to free-roam happily and safely before, you need to assess what is currently happening now, and it that is open to one rabbit potentially injuring the other, it cannot happen. The “being fine” when separated by a wall, but then aggressive when sharing a space is not unusual in unbonded rabbits.
There is a good outline of bonding steps to take and what signs to look for to go further in the RABBIT INFO section of this website. Be mindful of territorialness for your rabbits, as they both probably view the free-roam space as theirs, so being there together with not pen wall separating them is likely to cause territorial aggression in one and/or the other. Do you have neutral space you can work with? Typically people use their bathrooms!
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.