Frank says it would be an honour to fight Ken.
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.
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.
On the other hand, the instances of fighters like JBJ/Rashad or GSP/Condit I could imagine they do not train together regularly or haven't trained together long enough to form such a bond I would imagine they should
not have such a giant problem fighitng eachother.
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.
All in all, this is an interesting situation and has been discussed many times. I tend to agree with those who won't fight their teammates. In the UFC there should be enough depth in eahc division where this is not necessary (unless a title comes in the picture, in which case everything should go) and promoters should accomodate their fighters a little.