I think that you first need to make her understand that its childish to act like that. Not that she necessarily HAS to spend time with him, but at least get past whatever issues she's dealing with, so that she can at least be around him, and not put you in the middle of the 2 of them. It's not fair to YOU to have to choose between then, regardless of whether or not you're related, because as you said, you're all spiritual brothers and sisters.