Sarah C. had a great response about writing the letter and keeping it with your legal papers. He might not want to be as blunt about not agreeing with parenting styles and not liking the wife, though. You could position your decision around being considerate of the stress and strain of adding 2 more kids to their family suddenly, eg their house/cars may not be big enough, childcare may be an issue, their financial ability to care for and educate your kids if you're gone, etc. There are lots of other reasons you can give. It would be rather hurtful to be dealing with the loss and death of your brother and get a letter stating you're a lousy parent.
If your friends say they don't want to be guardians, it shouldn't cause things to be awkward in the friendship. Whatever reason(s) they give, it will be valid, even if it might seem selfish to you. You want people who have no hesitation in taking in your kids if you're not alive. If your friends don't feel they're up to the task, then accept it and feel glad that you're close enough that they could be honest with you.
It may be very stressful for you now, but at least you came to an agreement on your first choice for guardians!! I don't think you need to tell people or justify your decision about guardianship. Keep in mind that as your kids get older, circumstances may change and you might change your mind. For us, my in-laws are the guardians, but they live out of state. My husband and I know that as his parents get older and if their health changes, we will need to choose another guardian. We also have said that as our children get older, it will be harder to uproot them and have them move out of state. We reassess the guardianship issue every year.
This is tough to do, but be glad you're good to be facing these issues now. Good luck!