I would say this is an entirely differenty story. Ken and Frank: 1 aren't brothers. 2 Have a documented rivarly where there is real animosity between the two.
They are brothers for all intents and purposes. They had nobody and were joined under the same roof. The point is he said it would be a privelege/honour. He doesn't want to fight him to settle a score, he still loves his brother, he respects him as a martial artist.
I would imagine it differs on a case by case basis though when fighting teammates. In some cases like Josh Koshcheck/Jon Fitch where they have trained together for years or Shaub/Carwin where one literally has brought the other into the sport and mentored him I could see teammates being hesitant and opposed to fighting. The amount of respect for another and/or amount of tricks/specialties known can cause trepidation.
Honestly, that's their problem. Saying, 'I don't want to do something because I don't like it' doesn't mean its a good reason. I'm not saying they 'should' fight, again I think its silly. If you really value those close to you as fighters the best way to honour them is to teach them/learn from them ie. fight them.
Also, so what if they know all you're 'tricks'? You know theirs. Doesn't it make for a unique and challenging fight? It's an opportunity to grow past you're old reliances, make new tricks, anticipate his knowledge. After you fight them then you have twice a many tricks to use against unfamiliar opponents. You have grown your game.
The only real problem I see in such instances is who is cornered by which camp. It would foce coaches to pick "favorites" and leave the other fighter to quickly find another coach who doesn't know them as well and won't be able to corner them to the same extent as their regular trainer would be able to, putting them at a disadvantage.
How bout they don't rely on their old camp? How bout they go back into the world and see who has the best learning strategy, who picks the best teachers?
It would be totally fair because both fighter's are in the same position.
In my opinion, if gyms consider themselves to be professional, or have a martial arts ethic, they should be mature enough and intelligent enough to not only resolve an internal fight situation but to ACTUALLY EMBRACE IT, as an opportunity to grow their fighters and showcase their camps quality and professionalism.
All this tiptoeing around, avoiding eye contact, is essentially aversive
behavior, its immature, its emotional, 'I won't' vs 'I will', it is unbecoming of a martial artist.