Quite frankly, I think I would cut such a sibling out of my life(at least, until he or she grew up). There is no right to the absolute approval, endorsement and agreement of anyone else.
“and a mans enemies will be those of his own household. He who loves father or mother more than Me is not worthy of Me. And he who loves son or daughter more than Me is not worthy of Me.
http://www.biblegateway.com/passage/?search=Matthew%2010:36-37&version=NKJV
Perfect Scripture for the situation.
I agree with you. If it was my sister, tyrant behavior wouldn’t be tolerated. When she threatened to absent herself from the family if she didn’t get her way, my response would be, “Bye.”