In regard to the Al vs Mike comparison. I think one of the reasons Brian and Al are where they are is because sometimes it's just easier with a friend. There's less emotional baggage. Less pressure. There is an old saying, you can choose your friends, you can't choose your family. Al obviously has an easier going temperament which plays a role too.
Can anyone even imagine Mike getting away with what he says regularly about Brian if he were not a blood relative? If Mike were not related to Brian, Mike would still be trying to find a way to write songs with Brian, but Brian would have kicked Mike to the curb in a FAR bigger way many moons ago.