I can understand what he's doing, whether or not I agree with it. He's showing his disapproval for the sinful behaviour by not associating with you. Taken at face value, he's hoping that by his 'active' disapproval, you, (more your stepson) will realize that there's something wrong with the behaviour and correct it. This action taken to extremes is where monasteries began when monks began to retreat from the sin in the world at large. Whether they've been married previously or not, living together is NOT approved by the Bible, and is sinful, no matter how nice, sweet, religious or whatever the participants. Personally, I think engaging people in conversations is better than ostracising them, even if the conversations are painful and unpleasant. At least then there's a chance to express to someone WHY you disapprove of their actions. Christians are called to hate sin, but love sinners, and sometimes that can get hard and kinda foggy. Forgive your brother, tell him you love him anyway, and invite the conversation. Your 20 yr old stepson is big enough to handle his own battles, so let them talk. You're gonna be family forever, so this ought to be resolved before something tragic and irreparable occurs. Oh, remember that as a preacher, he has others watching him, espec. his church, and he must maintain an 'above reproach' lifestyle. He may be taking that to the extreme, but he is well-meaning.