Rabbits who have not yet reached the hormonal, puberty stages of growth often get along with other rabbits easily. It is likely this, rather than them being related that allowed them to get along. Rabbits do not acknowledge family units like we do — they will fight and mate with siblings and parents all the same, so the progression you are experiencing (i.e. they were getting along and now they are fighting) is actually quite normal and is to be expected now that they are older.
“Is this even possible to bond them now?” – Arguably, I’d say this marks when you can actually start bonding them. If you wanted to be extra cautious, you can wait a bit so there are no lingering effects of the neuter (post-neuter ‘craze’ as they call it) and to ensure there are no leftover grudges from previous negative interactions. For rabbits who were unable to get along post-puberty (which is a big portion of them, as yours did), you cannot start bonding until they are all spayed/neutered, since the procedure helps mitigate some of the challenges in creating lifelong bonds. That being said, neutering/spaying is not a magic fix — rabbits will still likely be territorial and aggressive, as you are seeing. Again, this is common and not surprising.
There is general information about the rabbit bonding process in the RABBIT INFO section of this website. It will start with introductions in neutral territory only (no where they have free-roamed themselves and claimed as their own space), as well as pre-bonding measures, like swapping what pens they are staying in or swapping their litter boxes and toys.
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.