Welcome to BB Mercedes.
First and most importantly, separating them immediately and making this permanent is the most important intervention you can do for their safety and well-being.
It is a very big misconception that rabbit siblings will get along. Mothers and children, sisters and brothers, etc. Rabbits do not recognize family units and do not have that sort of allegiance to each other. The reason this misconception is still around is because *baby* rabbits will get along with almost anyone because, as you mention, hormones are not in play. So it’s not the fact that they are siblings and have been together since birth, but it’s the fact that they are young and didn’t have hormones at the time. At this point, hormones have surged, therefore they can no longer be together until they are both spayed. We’ve had rabbits as young as 10 weeks old display hormonal behaviors on these forums. Hormones can develop early or late. It’s clear they are now present, so your rabbits need to be separated until a month following both their spays. They need to forget each other and the very bad interactions they’ve had, as rabbits can hold grudges.
You mention some bonding terminology, including neutral area, and you mention observing sniffing and general supervising, and that’s positive; however, these sort of bonding tactics can only be used for rabbits who are ready to bond. Since they have shown aggression, it’s more important that they both need to be spayed prior to them being able to do any proper bonding.
See https://binkybunny.com/BUNNYINFO/tabid/53/CategoryID/9/PID/940/Default.aspx for some fundamental bonding knowledge, which again, applies once they are both spayed.
You were misinformed and the seller should have been responsible to tell you they cannot be kept together. They potentially didn’t even know themselves. But what’s most important is that you’ve acknowledged something must be done immediately. Again, separate them until they are spayed. This may be months, but it will be well worth it for their happiness and health.
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.