The point the OP is trying to make, I think, is that there should be strongest family ties between very close relatives.
It's fair to expect a distant cousin to try and plot his way to your crown if he's got the claim for it, or for an Uncle to try and "persuade" you to switch to seniority so he has a shot at the throne.
But the brother you grew up with for years? Unless he's a jerk with conflicting traits, or you became rivals via some event, there should be some special respect between the two of you that prevents him jumping on the faction/plot bandwagon at the first opportunity!
Sure, you can keep him happy with gold and titles, or make sure he's harmless by dumping him in a minor holding or a church...but it feels a bit unrealistic that this is needed every single time...