As a father of four children, I resolved long ago that I would try to treat all my children equally. But in reality, I am always accused of favoritism. The one that feels he or she has been slighted, immediately throws accusations that I favor one over the other.
Siblings the world over have issues. Nothing new there. Individual circumstances and the degree of antipathy felt may differ, but still it is basically the same story everywhere. When the actions of one of your children are causing distress to another, what do you do? When you have already warned your child that following a particular course may not be morally correct because it may cause discord elsewhere. And still your child goes ahead with it and then the point arrives where what you were afraid of turns to reality.
But then who made me the arbiter of their morality? I am not God. There is a huge gap between how I see an issue and how they see it. Perhaps it is time for disengagement. We are talking about adults here. Most pain is caused because of the attachment you feel for your own. I should rather start disengaging, because I do not really need the pain anymore. They are adults and have their own set of views and approaches to their issues. Let them sort it out among themselves. Let them learn their lessons through their own heartaches, because I have had enough of those already.