Rhiannon: "How do you know?"

Violet: *points to her head* "I have a parasite."

She just did it and we got the explanation afterwards. Which plant she used, how she gave the poison to them, when and how she noticed that one of her opponents had an allergy.

We only found out later why she wanted to help with breakfast at all and when she could no longer help to prepare breakfast she was like Oh no i can no longer put poison in their food

There is a bunch of reasons why I think that theory is wrong, the main one being that the only actual similarity between the two sets of sibling is Violet and "the brother who made the skies surrender its power”. None of the other bonded a griffin, and as far as we know none of them is venin.

So, what we know about this story is this:

one brother bonded a dragon, one brother a griffin, and when the last one grew jealous he started drawing directly from the source.

I think we should consider this story exactly like that, a story, a fable and not take it too literallly.

Stop thinking for a second about this brothers has people, and consider them as figure of speech (maybe to pass down more erasily even to children the story of the continent)

So what if the three brothers were the nations that make up the continent? Because if you look at the story from this perspective, then you do have three "brother" who have the same characteristic as the ones in the story:

Navarre is the brother who bonded dragons;

Poromiel is the brother who bonded griffins;

The Barrens is the brother who got jealous and turned venin;

And I promise that if you look at the hints scattered inside the books you'll see that:

1. Dragons are endemic to the territories that make up Navarre. Even the original hatching grounds are located inside what is now Navarre (suspiciously close to the border of the wards.. that's another theory though);

2. Griffins are endemic to Poromiel, their "hatching grounds" (not sure we can use the same terminology) is in the area where Cliffsbane academy is/was (we learn that when the griffin riders share their process of entering the college);

3. The Barrens have no mythical / magical creatures that we know off.

If you think about it, at one point in history Poromielians (how are they called lol ??) and Navarrians bonded mythical / magical creatures that gave them magic, on top of any military or economical advantages they might had. It is understandable how the people of the Barrens might have felt threatened and searched for alternatives to the newfound magic of their neighbors.

We then know that the Venins originated from the Barrens, so that's the solution they found.

The story also tells us that:

The younger brother made the skies surrender its greatest power and thus vanquished his jealous brother at a great and terrible price.

Which again parallels the fact that Navarre unified, and was created as a nation, only after the war against the Venins 650 yars ago, therefore it makes sense to be reffered as the younger brother.