Utopia,
If this person were not a sibling, not a relative..........would they be the kind of person you would include in your life? Would you still expend the same amount of energy on the relationship, or would you simply say, "Poor fit for me, see you later" and let them get on with their lives?
I've driven myself near to insanity trying to make some siblings/relatives see what I want them to see.
Letting them go -- to believe what they want, to live how they want -- has provided such a refreshing serenity.